url: fix incomplete $-numeric behavior

This

    $10/bar.txt

was partially matching but should not match at all.

This commit fixes this by simply `[\w]` to `[A-Za-z_]` as the first
character of a bare relative path, so digit-starting fragments can't
match.

Another option would be a lookbehind but I think the check above is much
simpler.

src/config/url.zig

@@ -97,7 +97,7 @@ const rooted_or_relative_path_branch =
 
 // Branch 3: Bare relative paths such as src/config/url.zig.
 const bare_relative_path_prefix =
-    \\[\w][\w\-.]*\/
+    \\[A-Za-z_][\w\-.]*\/
 ;
 
 const bare_relative_path_branch =
@@ -446,6 +446,7 @@ test "url regex" {
         // $-numeric character should not match
         "$10/bar",
         "$10/$20",
+        "$10/bar.txt",
     };
     for (no_match_cases) |input| {
         var result = re.search(input, .{});