Pin @handlebars/parser to 2.0.x.

[rwjblue]

Updating to @handlebars/[email protected] introduces a number of test failures that need to be debugged:

not ok 1378 Chrome 89.0 - [2 ms] - [integration] jit :: {{log}} keyword: correctly logs `this`
    ---
        actual: >
            null
        stack: >
            TypeError: Cannot read property 'length' of undefined
                at ExpressionNormalizer.path (http://localhost:7357/153311786233/tests/assets/glimmer-vm.js:38770:23)
                at ExpressionNormalizer.normalize (http://localhost:7357/153311786233/tests/assets/glimmer-vm.js:38746:23)
                at http://localhost:7357/153311786233/tests/assets/glimmer-vm.js:38795:22
                at Array.map (<anonymous>)
                at ExpressionNormalizer.callParts (http://localhost:7357/153311786233/tests/assets/glimmer-vm.js:38794:30)
                at StatementNormalizer.MustacheStatement (http://localhost:7357/153311786233/tests/assets/glimmer-vm.js:38956:33)
                at StatementNormalizer.normalize (http://localhost:7357/153311786233/tests/assets/glimmer-vm.js:38900:23)
                at http://localhost:7357/153311786233/tests/assets/glimmer-vm.js:38625:25
                at Array.map (<anonymous>)
                at Object.normalize (http://localhost:7357/153311786233/tests/assets/glimmer-vm.js:38624:67)
        message: >
            Died on test #2     at _loop (http://localhost:7357/153311786233/tests/assets/glimmer-vm.js:17888:21)
                at http://localhost:7357/153311786233/tests/assets/glimmer-vm.js:17898:11
                at suite (http://localhost:7357/153311786233/tests/assets/glimmer-vm.js:17900:9)
                at jitSuite (http://localhost:7357/153311786233/tests/assets/glimmer-vm.js:17814:12)
                at Module.callback (http://localhost:7357/153311786233/tests/assets/tests.js:9760:18)
                at Module.exports (http://localhost:7357/153311786233/tests/assets/loader.js:106:32)
                at requireModule (http://localhost:7357/153311786233/tests/assets/loader.js:27:18)
                at http://localhost:7357/153311786233/tests/index.html?hidepassed:140:11: Cannot read property 'length' of undefined
        negative: >
            false
        browser log: |

not ok 1415 Chrome 89.0 - [3 ms] - [integration] jit :: modifiers: modifiers on components accept `this` in both positional params and named arguments, and updates when it changes
    ---
        actual: >
            null
        stack: >
            TypeError: Cannot read property 'length' of undefined
                at ExpressionNormalizer.path (http://localhost:7357/153311786233/tests/assets/glimmer-vm.js:38770:23)
                at ExpressionNormalizer.normalize (http://localhost:7357/153311786233/tests/assets/glimmer-vm.js:38746:23)
                at http://localhost:7357/153311786233/tests/assets/glimmer-vm.js:38795:22
                at Array.map (<anonymous>)
                at ExpressionNormalizer.callParts (http://localhost:7357/153311786233/tests/assets/glimmer-vm.js:38794:30)
                at ElementNormalizer.modifier (http://localhost:7357/153311786233/tests/assets/glimmer-vm.js:39108:33)
                at http://localhost:7357/153311786233/tests/assets/glimmer-vm.js:39059:23
                at Array.map (<anonymous>)
                at ElementNormalizer.ElementNode (http://localhost:7357/153311786233/tests/assets/glimmer-vm.js:39058:41)
                at StatementNormalizer.normalize (http://localhost:7357/153311786233/tests/assets/glimmer-vm.js:38897:52)
        message: >
            Died on test #2     at _loop (http://localhost:7357/153311786233/tests/assets/glimmer-vm.js:17888:21)
                at http://localhost:7357/153311786233/tests/assets/glimmer-vm.js:17898:11
                at suite (http://localhost:7357/153311786233/tests/assets/glimmer-vm.js:17900:9)
                at jitSuite (http://localhost:7357/153311786233/tests/assets/glimmer-vm.js:17814:12)
                at Module.callback (http://localhost:7357/153311786233/tests/assets/tests.js:11623:18)
                at Module.exports (http://localhost:7357/153311786233/tests/assets/loader.js:106:32)
                at requireModule (http://localhost:7357/153311786233/tests/assets/loader.js:27:18)
                at http://localhost:7357/153311786233/tests/index.html?hidepassed:140:11: Cannot read property 'length' of undefined
        negative: >
            false
        browser log: |
    ...

These failures were introduced by the changes in handlebars-lang/handlebars-parser#4, but it is unclear exactly why they are failing. Someone needs to investigate.

[rwjblue]

I've created #1286 to track unpinning.