perf(help): generate helptags with LPeg

Problem: Generating helptags builds and queries a full vimdoc syntax
tree. This is much slower than the previous scanner and also happens
before discovering that the tags file cannot be written.

Solution: Extract tags with an LPeg grammar, reading each file once and
skipping example blocks and ordinary text. Open the output first so
unwritable directories can return immediately.

This restores legacy handling of tags inside inline code spans. On an
arm64 Mac, the 5.36 MB runtime corpus takes 24 ms instead of 604 ms,
with identical generated tags.

AI-assisted

0 files changed, 0 insertions, 0 deletions