Add Phpactor as alternative PHP language server

[LakshyAAAgrawal]

Context

PR #140 adds PHP support via Intelephense (npm-based). Intelephense works well but is proprietary (free tier covers core LSP features, premium features like rename/code actions require a $35 license).

Phpactor is a fully open-source (MIT) alternative that supports all LSP features multilspy needs:

• textDocument/definition
• textDocument/references (cross-file)
• textDocument/completion
• textDocument/hover
• textDocument/documentSymbol
• Plus: rename, code actions, go-to-implementation — all free

Trade-offs vs Intelephense

	Intelephense	Phpactor
License	Proprietary (free tier)	MIT
Install	npm (Node.js)	PHAR download or composer (PHP 8.1+)
Rename/code actions	Paid	Free
Cross-file refs	Free	Free
Runtime dependency	Node.js	PHP 8.1+

Implementation notes

• Could be added as or selected via config
• PHAR binary can be downloaded from GitHub releases
• Launches with (stdio by default)
• Requires PHP 8.1+ on the host