Extract MySQL lexer and parser polyfills

[adamziel]

Summary

• move the existing PHP MySQL lexer implementation into WP_MySQL_Polyfill_Lexer
• move the existing PHP MySQL parser implementation into WP_MySQL_Polyfill_Parser
• keep the public WP_MySQL_Lexer and WP_MySQL_Parser classes as thin subclasses of the polyfills

Testing

• php -l on changed PHP files
• parser smoke: lex and parse SELECT 1 through the public PHP classes

This PR is intentionally extraction-only. Native routing and parser-node behavior changes are stacked in follow-up PRs so their diffs are obvious.