Mark js-assignment functions' symbols as methods

[sandersn]

This is a better fix than #23068 and #22935 because it makes js-assigned functions work more like methods, so they reuse more of the existing method machinery.

This make display more accurate:

1. The functions now print as methods
2. When the type of the function references the class, the type of the function itself is now printed correctly instead of as any.

Other benefits:

1. isMethodLike no longer needs to have a js special case.
2. The check in convertFunctionToEs6Class makes more sense.

Downsides:

1. Both getTypeOfFunc and getTypeOfVariable need js special cases, as does getSignatureOfSymbol.