feat: Add support for destructuring locals

[ExE-Boss]

Supersedes and closes #450.

Destructured locals have to be specified at compile time. All other locals need to be accessed via locals (or whatever ejs.localsName is set to).

This works for KumaScript, where the only locals are API namespaces and macro arguments $$ (all arguments array) and $0-$9, which don’t change between macros or API invocations.

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review?(@mde)

[mde]

This is pretty great. :) Couple of issues:

• Since this is opt-in, wouldn't it make more sense just to destructure everything in locals, as a strict-mode safe alternative to using with? Why bother asking the user to specify particular keys?

• Could we get a test that involves multiple destructured variables, instead of just one key?

Otherwise awesome. This looks like a really useful path forward to avoid verbosity without having to resort to using with.

[ExE-Boss]

This is because locals are passed at execution time, whereas the locals to destructure are specified at compile time.

Because of this, it’s impossible to know what exactly locals will have at execution time, so you have to specify variables to destructure at compile time.

And if you’re only compiling, immediately executing and discarding code, you can always just do Object.keys(locals).

[mde]

Oh, duh, sorry. I tend to overlook the precompile use-case. This makes good sense. If you could beef up the tests a bit, and update the README, I'll be happy to merge this, and then plug the same documentation into the EJS site.

[mde]

LGTM. Thanks for this!