Remove validation of mustache statements in no-invalid-aria-attributes rule

[judithhinlung]

This PR fixes #2405 and fixes #2451.

Besides string, number, and boolean literals, mustache statements often also contains helpers, variables, and other nested statements. Since it is not possible to check what a given mustache statement evaluates to at the template level, this PR removes their validation to prevent valid ARIA attribute values in complex statements from failing the rule.

Allowed:

# examples
<div aria-label={{this.foo}} />
<div aria-label={{if @label @label @placeholder}} />
<button aria-label={{if @isNew (t "actions.add") (t "actions.edit")}}></button>

Testing

Test Suites: 135 passed, 135 total
Tests: 5 skipped, 7459 passed, 7464 total
Snapshots: 969 passed, 969 total
Time: 96.051 s
Ran all test suites.
✨ Done in 107.73s.

[bmish]

Thanks!