feat: support pie

[zeriyoshi]

• Reviewed feedback about Composer package name validation
• Add lowercase conversion for EXT_NAME and EXT_VENDOR (e.g. "MyExt" → "myext")
• Add character validation to reject invalid Composer package name characters (spaces, slashes, @, etc.)
• Run code review and security checks

[Copilot]

Pull request overview

This PR updates the extension skeleton initialization workflow to better support PIE/Composer-style packaging and improves repository hygiene for generated extension artifacts.

Changes:

• Extend pskel init to accept an optional vendor_name, and (for PHP < 8.5) generate a composer.json for the new extension.
• Add ext/.gitignore to avoid committing common build/autotools artifacts produced inside /ext.
• Add .gitattributes export-ignore rules and update devcontainer configs to symlink the workspace pskel.sh into /usr/local/bin/pskel.

Reviewed changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
pskel.sh	Parses optional vendor, invokes ext_skel.php accordingly, and generates composer.json for PHP < 8.5.
ext/.gitkeep	Normalizes the marker content for uninitialized ext/.
ext/.gitignore	Ignores common generated artifacts inside the extension working directory.
.gitattributes	Defines export-ignore rules for producing a cleaner git archive.
.devcontainer/local/devcontainer.json	Adds postCreateCommand to symlink pskel to the workspace script.
.devcontainer/devcontainer.json	Same as above for the Codespaces devcontainer definition.

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