[JavaScript] Clean up class member code

[Thom1729]

Inspired by improvements from #3848.

Consolidate the code handling class members (fields and methods) into a more coherent and stack-based approach.

• Handle “backtrackable” elements (modifiers, accessor keywords, async) in a self-contained way in the method-declaration stack.
• As a side benefit of the above, remove the duplicate get/set/async code that was needed due to Allow a syntax rule to fail multiple branch points. sublime_text#3494.
• Also expand function scopes to include modifiers.
• Explicitly handle semicolons in class field declarations, letting us mark extraneous semicolons punctuation.terminator.statement.empty.js like in statement contexts. (This has proved useful for testing in the past.)
• Remove static-block-body, which can be replaced with the regular block context.
• Various minor improvements.