feat: add php-prebuilt plugin - #1188

Summary

Add the php-prebuilt shortname for a second, complementary PHP plugin: prebuilt, relocatable PHP binaries for frozen EOL versions (5.4.45 through 8.1.34) on linux-x86_64.

• Tool repo URL: https://github.com/php/php-src
• Plugin repo URL: https://github.com/x-rush/asdf-php

This does not change plugins/php. asdf-community/asdf-php is actively maintained and stays the default; it compiles PHP from source via php-build. The new plugin solves a different problem: legacy PHP that no longer compiles on a current distro because of OpenSSL / bison / libxml2 / ICU drift. It installs a checksum-verified, self-contained artifact instead. Users opt in explicitly:

asdf plugin add php-prebuilt
asdf install php-prebuilt 7.4.33
asdf local php-prebuilt 7.4.33

The plugin name is the tool name in asdf, so .tool-versions must use php-prebuilt. Both migration paths have been verified against asdf 0.20.2 and documented in the plugin README.

Scope is deliberately narrow: no macOS and no arm64 artifacts have been built, and no PHP 8.2+ versions are offered. The plugin README states those limits up front.

Checklist

• Format with scripts/format.bash — the README row is aligned to the existing table and passes Prettier on an LF-normalized copy. No shell scripts were added, so shfmt is not applicable.
• Test locally with scripts/test_plugin.bash --file plugins/php-prebuilt — OK plugins/php-prebuilt.
• Your plugin CI tests are green — x-rush/asdf-php run 42501403364: the contract suite (65 checks) plus a real asdf-vm/actions/plugin-test@v4 job that clones the plugin, adds it, installs 7.4.33 from a published artifact and runs php -v. The plugin implements all three required callbacks (list-all, download, install).