Add php's php-language-server to built-in guessed servers

Add php's php-language-server to built-in guessed servers

Closes https://github.com/joaotavora/eglot/issues/1. The problem in that issue is that php-language-server has a
bug when it's not passed it the deprecated ":rootPath" field. The bug
doesn't happen if the ":processId" field is also absent. Eglot was
triggering the bug, because it didn't pass ":rootPath", but there's
nothing wrong in doing so.

* README.md: Add php-language-server to the built-in list.

* eglot.el (eglot-server-programs): Add php-language-server.
(eglot--connect): Also pass (deprecated) rootPath.