fix: robustly strip psql meta commands

[ignat980]

fix(compiler): robustly strip psql meta commands without breaking SQL

Replace naive line-based removal with a single-pass state machine that correctly distinguishes psql meta-commands from backslashes in SQL code, literals, and comments.

The previous implementation would incorrectly strip any line starting with a backslash, breaking valid SQL containing:

• Backslashes in string literals (e.g. E'\\n', escape sequences)
• Meta-command text in comments or documentation
• Dollar-quoted function bodies with backslash content

Changes:

• Track parsing state for single quotes, dollar quotes, and block comments
• Only remove backslash commands at true line starts outside any literal context
• Properly handle escaped quotes (''), nested block comments (/* /* */ */)
• Support dollar-quoted tags with identifiers (e.g. $tag$...$tag$)
• Add comprehensive test suite covering:
  • All documented psql meta-commands (\connect, \set, \d*, etc.) See PostgreSQL psql docs
  • String literals with backslashes and nested quotes
  • Dollar-quoted blocks with various tag formats
  • Nested block comments containing meta-command text
  • Edge cases: empty input, whitespace-only, missing newlines

Performance improvements:

• Pre-allocate output buffer with strings.Builder.Grow()
• Single pass eliminates redundant string operations
• Reduces allocations by avoiding intermediate line slices

Testing

• go test ./internal/compiler
• 100% test coverage of new function removePsqlMetaCommands()

Addresses gbarr's comment in #4082 which closes #4065

[ignat980]

@andrewmbenton please review

[gbarr]

Thanks @ignat980 this looks a much more complete solution than I was expecting

[kyleconroy]

@ignat980 if you open this against main I can get this merged. Just make sure to includes Andrew's commits.