amp-bind: Improve init performance

[dreamofabear]

Related to #6199.

Several slow tasks are necessary during initialization. We should speed it up where possible and chunk/amortize across multiple frames otherwise:

• DOM scan for bindings
• Expression parsing and AST generation

[dreamofabear]

Re: DOM scan for bindings

Did some profiling and it looks like using an attribute selector with querySelectorAll is faster by roughly the proportion of bound elements to all elements.

<p bindable [attr]="expr"></p>

const boundElements = document.body.querySelectorAll('[bindable]');

However, this is less user friendly and still takes several ms for documents with thousands of elements/attributes, which may not fit in a single frame's budget.

For both cases, the majority (> 90%) of the time is spent iterating over elements and attributes. So as a first step, I'll chunk this part and set a budget of X ms for a single frame.