Add preprocessEmbeddedTemplates function

[pzuraq]

Adds a function for preprocessing embedded templates. This function finds embedded templates using a simple RegExp + stack based parser which is very limited overall and designed specifically for this purpose. It replaces the embedded templates with parseable JS, and also pre-parses and walks the templates to find all possible identifiers within them and exposes them externally:

class Foo {
  <template>
    <Bar/>

    {{baz}}
  </template>
}

Becomes:

class Foo {
  [GLIMMER_TEMPLATE(`
    <Bar/>

    {{baz}}
  `, { scope: () => ({ Bar, baz }) })];
}

The preprocessor does not understand the surrounding JS scope and so does not know if the identifiers it emits actually exist in scope and are valid (it will only emit valid JS identifiers, however). So, if the preprocessed output is going to be used by another tool, such as a linter, it's up to the user to postprocess the results and remove errors pertaining to this generated code.

The result also includes a list of replacements which let the user know what was replaced in the original template, useful for fixing linting failure locations and so on. Also, it can include and handle sourcemaps.

[rwjblue]

Discussed this a bit in discord:

• Let's add an API to @glimmer/syntax to effectively take either a string template or template AST and return a list of the locals
• Update this to leverage that API (which can be retrieved from Ember's ember-template-compiler.js).
• Figure out a sync API (we'll need the same general code for eslint plugins)