feat[lang]: native hex string literals

[tserg]

What I did

Resolves #4206

How I did it

Add a custom parser to watch for STRING tokens whenever there is a NAME tokens of "x" value, store the location of these hex literals, and transform them to byte values during transformation of the python ast to vyper ast.

How to verify it

See new tests.

Commit message

feat: native hex string literals

This PR adds a custom parser to track the locations
of native hex literals, which are converted to byte
values during transformation of the python ast to
vyper ast.

Description for the changelog

Cute Animal Picture

[charles-cooper]

maybe just ParserState

[charles-cooper]

please add docs!