Handle SQL strings containing escaped quotes

[Denchick]

Summary

• preprocess lexer input to replace doubled single quotes with placeholders so the Ragel scanner no longer terminates a string literal early, and unescape the placeholders when emitting SCONST tokens
• add a regression test that parses a SELECT with escaped single quotes to prove the lexer accepts embedded quotes and semicolons inside literals

Testing

• make test

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Codex Task