fix(completion): complete drive-letter filepath on Windows #36353

Problem:
On MSWIN, file completion (CTRL-X CTRL-F) only works for the current
drive (so not for actual absolute paths), since drive letters are never
included in the completion pattern.

e.g. when completing "F:\Hello" Nvim currently completes "\Hello"
which is relative to the current drive/volume.

vim solves this by adding ':' to the default 'isfname' value on mswin,
but that causes issues as ':' is not a valid windows path char anywhere
_except_ after the drive letter.

Solution:
detect drive letters in front of the path when creating the completion
pattern.

src/nvim/file_search.c

@@ -1679,7 +1679,7 @@ char *file_name_in_line(char *line, int col, int options, int count, char *rel_f
 
   // Search forward for the last char of the file name.
   // Also allow ":/" when ':' is not in 'isfname'.
-  len = path_has_drive_letter(ptr) ? 2 : 0;
+  len = path_has_drive_letter(ptr, strlen(ptr)) ? 2 : 0;
   while (vim_isfilec((uint8_t)ptr[len]) || (ptr[len] == '\\' && ptr[len + 1] == ' ')
          || ((options & FNAME_HYP) && path_is_url(ptr + len))
          || (is_url && vim_strchr(":?&=", (uint8_t)ptr[len]) != NULL)) {