ci: add PR auto-labeler

[hasansezertasan]

Context

Adopt PR auto-labeling (from #16) using fuxingloh/multi-labeler driven by a .github/labeler.yml config, plus a job that tags PRs internal/external.

This is the adapt-heavy item: PR #16's 306-line labeler.yml is tailored to the litestar monorepo (test/unit, test/e2e, v3 base branch, …) and the internal/external job uses a hardcoded maintainer list. Both need generalizing for a template.

Proposed tasks

• Add .github/workflows/pr-labeler.yml (SHA-pinned, least-privilege) + .github/labeler.yml
• Write a template-appropriate labeler.yml:
  • type/* from Conventional Commit title prefixes (feat/fix/docs/…)
  • area/* from generic paths (src/**, tests/**, docs/**, .github/**)
  • Breaking from ! in the title
• Create the labels the config references (they don't exist in this repo yet)
• Decide the internal/external approach: replace the hardcoded maintainer list with org-team membership (e.g. @litestar-org/members) or drop that job
• Ensure it passes the zizmor gate (ci: adopt zizmor for GitHub Actions security #22/ci: adopt zizmor and harden GitHub Actions workflows #39)

Notes

• Safe use of pull_request_target: label-only actions, no checkout/execution of PR head code.
• Pairs with ci: adopt zizmor for GitHub Actions security #22's existing repo labels and the CODEOWNERS teams from docs: add CODEOWNERS #26.