Eglot: unbreak eglot-extend-to-xref on w32

Eglot: unbreak eglot-extend-to-xref on w32

Because of a drive-letter casing mismatch between 'buffer-file-name'
and the return value of 'url-generic-parse-url', the hash-table test
'equal' in 'eglot-current-server' failed.

This failed to recognize that the file xref landed us on really is
managed by the language server that facilitated that jump.

The function w32-long-file-name seems to convert "C:/Users/" to
"c:/Users" consistently and so is a good addition to
eglot--uri-to-path.

* lisp/progmodes/eglot.el (eglot--uri-to-path): Use
w32-long-file-name.