Lint enum usage so that they are fully DCE-able

### Description
Context: https://github.com/ampproject/amphtml/issues/36453

Currently, enum objects are left in the output binaries because we're using them as dynamic object values (eg, loop keys or performing dynamic `Enum[key]` lookups). In an ideal codebase, enums would be used via `Enum.keyName` lookups only, which means that the value associated with that key can be inlined directly in to output code.

We suspect this is one of the last issue with that causes Terser's outputs to be larger than Closure compiler.