chore: Give fork PRs a clear create-vsix failure message

[TwitchBronBron]

Problem

Applying the create-vsix label to a PR opened from a fork starts this job, but GitHub withholds repository secrets from fork-triggered runs. The run then died with a cryptic:

Input required and not supplied: app_id

Nothing in that message says "this is a fork" or "here's what to do instead", so it reads like a broken workflow rather than an expected limitation.

Change

Detect the fork up front and fail immediately with an actionable message:

# GitHub withholds repository secrets from fork-triggered runs, so fail fast with a clear message
- name: Reject fork PRs
  if: github.event.pull_request.head.repo.fork
  run: |
    echo "::error::Could not build a .vsix because fork PRs can't access the required secrets. Ask a maintainer to dispatch it manually for PR #${{ github.event.pull_request.number }}: https://github.com/${{ github.repository }}/actions/workflows/create-vsix.yml"
    exit 1

Notes:

• Uses ::error:: so the text surfaces as an annotation in the Actions UI, not just in the log.
• The PR number and workflow link are built from context, so the step is identical across every repo.
• On workflow_dispatch there's no pull_request payload, so the if is falsy and the step skips — that's the path a maintainer uses to build a fork PR's .vsix.
• Runs as the first step, so a fork PR fails in seconds.

Matches rokucommunity/vscode-brightscript-language#864, where the message and annotation rendering were verified on a live run.

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