enh(go) add min, max, and clear built-in functions (Go 1.21)

[dylanpulver]

Changes

src/languages/go.js's BUILT_INS list is missing the three built-in functions Go added in 1.21 (Aug 2023): clear, max, and min. The Go spec's "Built-in functions" section lists all of append, cap, clear, close, complex, copy, delete, imag, len, make, max, min, new, panic, print, println, real, recover; the grammar has every one except those three.

Effect on current main — min, max, clear are highlighted as plain identifiers, while len/append (also predeclared, and already in the list) get built_in:

clear(s)          // not highlighted
lo := min(1, 2)   // not highlighted
n  := len(s)      // hljs-built_in  (control)

Fix adds the three names to BUILT_INS, in their alphabetical slots. They're predeclared identifiers (shadowable), exactly like the len/cap/append entries already treated as built_in, so this is consistent with existing behaviour.

Added a test/markup/go/builtins fixture covering the three new builtins plus len/append as controls; it fails on main (only clear/min/max differ) and passes with the change. Full go markup suite green, lint-languages clean.

Checklist

• Added markup tests, or they don't apply here because...
• I have read and followed our AI-assisted contributions policy (human review, no slop)

Assisted-by: Claude Opus 5 (high)