fix(path): restore space separation in 'path' (#25571)

Removing this behavior causes more inconsistencies and bugs.

src/nvim/options.lua

@@ -5926,6 +5926,10 @@ return {
         option may be relative or absolute.
         - Use commas to separate directory names: >
         	:set path=.,/usr/local/include,/usr/include
+        <	- Spaces can also be used to separate directory names.  To have a
+          space in a directory name, precede it with an extra backslash, and
+          escape the space: >
+        	:set path=.,/dir/with\\\ space
         <	- To include a comma in a directory name precede it with an extra
           backslash: >
         	:set path=.,/dir/with\\,comma

src/nvim/path.c

@@ -848,7 +848,7 @@ static void expand_path_option(char *curdir, garray_T *gap)
   char *buf = xmalloc(MAXPATHL);
 
   while (*path_option != NUL) {
-    copy_option_part(&path_option, buf, MAXPATHL, ",");
+    copy_option_part(&path_option, buf, MAXPATHL, " ,");
 
     if (buf[0] == '.' && (buf[1] == NUL || vim_ispathsep(buf[1]))) {
       // Relative to current buffer: