Detect lexical ambiguity due to regex overlap

[Scott-Guest]

If two regex terminals overlap and have the same prec(_) value, their precedence in the lexer is flaky and dependent on set iteration order (see #4255).

We should detect such ambiguities, either during every kompilation if it can be done cheaply, or with a dedicated -Wregex-overlap flag.

[Scott-Guest]

Blocked on #4266. If we decide to allow negative lookbehind/lookahead, this becomes undecidable.

[Scott-Guest]

Blocked on #4295. We need a structured representation in order to translate to the dk.brics.automaton syntax.

[Scott-Guest]

Also consider non-regular expression tokens. We may need to add prec attributes to tokens as well then

[Scott-Guest]

Also consider user list separators