fix: canonicalize keywords

[Snehil-Shah]

Currently, many keyword values are stored as direct input values in the AST which later also produces a different digest for different casings.

This PR now stores the canonical spelling at parse time instead of deriving from input, resulting in stable ASTs and stable digests.

It also adds dialect-aware canonicalization to function names and how their semantics change when they are delimited, across both MySQL and PgSQL.

[coderabbitai]

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[Snehil-Shah]

@renecannao I added a dedicated file as an AST-level regression suite for this. Let me know if you want it spread across existing files instead.