This is a simple extension of GenericLexer, and is used in more than
just LibXML, so let's move it into AK.
The move also resolves a FIXME, which is removed in this commit.
Due to the way expression parser is written, we need to resolve the
ambiguity between member access operators and dots used for punctuation
during lexing. The lexer uses a (totally bulletproof) heuristic to do
that: whenever '.' is followed by ' ' or '\n', it is considered a dot
and member access otherwise. While it works fine for prettified test
cases, non-prettified files often lack enter after a trailing dot
character. Since MemberAccess will always be invalid at that position,
explicitly treat trailing dot as a part of punctuation.
