Lexical "this" capture

[ef4]

Opening for discussion on whether this is a good direction to solve emberjs/babel-plugin-ember-template-compilation#61

You can see the other half of the changes in:

emberjs/babel-plugin-ember-template-compilation#67

which patches this PR's changes into ember-template-compiler.js.

The idea here is that @glimmer/syntax continues to just trust the outer layer (babel-plugin-ember-template-compilation) to manage all aspects of JS scope, including whether or not this should be considered to in local scope. If it's in scope, it compiles to a normal variable reference.

[github-actions]

	Dev	Prod
This PR	676K └─┬ . 191K ├── runtime 162K ├── syntax 124K ├── compiler 81K ├── opcode-compiler 49K ├── manager 19K ├── validator 11K ├── program 9.0K ├── reference 7.0K ├── destroyable 6.1K ├── util 4.2K ├── node 3.8K ├── global-context 2.6K ├── wire-format 1.0K ├── vm 966B ├── encoder 844B ├── vm-babel-plugins 565B └── owner	262K └─┬ . 71K ├── runtime 69K ├── syntax 56K ├── compiler 26K ├── opcode-compiler 15K ├── manager 5.0K ├── program 4.1K ├── validator 3.7K ├── reference 2.4K ├── util 2.1K ├── node 1.6K ├── wire-format 1.5K ├── destroyable 737B ├── vm 563B ├── global-context 511B ├── encoder 469B ├── vm-babel-plugins 155B └── owner

[ef4]

Was discussed and approved during last week's spec meeting.