highlight + scope tree fixes for 'operator', 'const', 'noexcept'

A couple more fixes + tweaks:

1. Member functions with `const`, `const noexcept()` were not properly added in the scope tree.
2. More tweaks for `operator` function definitions; this is mainly done by rolling `operator X` into a single symbol. This also provides syntax highlighting that might be a bit contentious (IMO it makes things easier to read for e.g. definitions of `operator()`, though)