Dynamically-scoped variable accessors

[ef4]

This adds the expressison syntax {{-get-dynamic-var "yourVariableName"}} and the statement syntax {{#-with-dynamic-var "yourVariableName" someValue}}...{{/with-dynamic-var}}.

It's a blocker for a glimmer2-compatible liquid-fire, because we need to read and write the outletState variable.

These are both dash-prefixed to mark them private, but emberjs/rfcs#95 discusses making them public.

This is work-in-progress until @chancancode reviews and we simplify.

[ef4]

Tests broke when I rebased. 640c4e9 is the culprit. Investigating.

[ef4]

Ah, my tests were using with-keywords, which was removed.

with-dynamic-var itself is probably the appropriate replacement.

[rwjblue]

The current build failures look like typescript validation issues. You should be able to replicate with npm run vscode-build.

[ef4]

What is the appropriate type for this to return? ValueReference<any> is no good because UNDEFINED_REFERENCE is not of that type.

  evaluate(vm: VM): ValueReference<any> {
    let scope = vm.dynamicScope();
    if (scope.hasOwnProperty(this.varName)) {
      return scope[this.varName];
    } else {
      return UNDEFINED_REFERENCE;
    }
  }

[ef4]

Looks like maybe PathReference<Opaque> is right, based solely on pattern-matching other places in the codebase.

[ef4]

Is this cast to <any> appropriate, or is there a better way to do this?

[chancancode]

Closed in #297