fix(plpgsql-deparser): preserve ALIAS FOR declarations

[pyramation]

Summary

Closes the last PL/pgSQL round-trip gap: ALIAS FOR declarations were dropped on deparse, so bodies referenced undeclared names:

DECLARE arg ALIAS FOR $1;   -- input
BEGIN RETURN arg + 1; END    -- old output: no declaration, `arg` undefined

The parser half already shipped — libpg-query@18.1.2 (which this repo now consumes) serializes aliases on PLpgSQL_function:

"aliases":[{"PLpgSQL_alias":{"name":"arg","varno":0,"lineno":2}}]

This PR is the deparser half (nothing else renders it):

• new PLpgSQL_alias type ({name, varno, lineno?}) on PLpgSQL_function.aliases
• deparseDeclareSection emits name ALIAS FOR <datums[varno].refname>; interleaved with variable declarations by source lineno (variables win ties, so an alias of a local variable follows its target's declaration)
• fixtures 63–65 (positional-param, named-param + local-var interleaving, old/new trigger aliases) + snapshot tests

Note: the DECLARE section now round-trips exactly; a RETURN <alias> body still deparses to the alias's target name (e.g. RETURN buffer / RETURN NEW) because the RETURN carries retvarno pointing at the underlying datum — semantically identical and AST-round-trip-clean, just not textually the alias.

Supersedes #308 (rebuilt fresh on current PG18 main; #308 can be closed).

Local: plpgsql-deparser 109, plpgsql-parser 303, plpgsql-parse 9, parse 68 — all green against the published 18.1.2 parser.

Link to Devin session: https://app.devin.ai/sessions/eeb8deba0c04475091df703c88877302
Requested by: @pyramation

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