refactor(path)!: support RFC3986 in path_with_url() #36564

Problem:
Nvim does not recognize URI scheme with numeric characters. While rare, there
are URIs that contain numbers (e.g. [ed2k://](https://en.wikipedia.org/wiki/Ed2k_URI_scheme))
and characters like `+` (e.g. `svn+ssh`). I use it in
[distant.nvim](https://github.com/chipsenkbeil/distant.nvim) to support
multiple, distinct connections using `distant+1234://` as the scheme.
Otherwise, if you open a file with the same name & path on two different
machines from the same Nvim instance, their buffer names will conflict
when just using `distant://`.

Solution:
Adds full support for detecting URI scheme per
[RFC3986](https://www.rfc-editor.org/rfc/rfc3986#section-3.1)

test/unit/path_spec.lua

@@ -693,20 +693,51 @@ describe('path.c', function()
   end)
 
   describe('path_with_url', function()
-    itp('scheme is alpha and inner hyphen only', function()
+    itp('scheme is alpha and inner numeric, "+", "-", "." only', function()
       local function path_with_url(fname)
         return cimp.path_with_url(to_cstr(fname))
       end
+
+      -- Check normal scheme with just alphabetic
       eq(1, path_with_url([[test://xyz/foo/b0]]))
       eq(2, path_with_url([[test:\\xyz\foo\b0]]))
-      eq(0, path_with_url([[test+abc://xyz/foo/b1]]))
+
+      -- Check valid scheme with just alphanumeric
+      eq(1, path_with_url([[test123://xyz/foo/b0]]))
+      eq(2, path_with_url([[test123:\\xyz\foo\b0]]))
+
+      -- Check invalid scheme (contains invalid character)
       eq(0, path_with_url([[test_abc://xyz/foo/b2]]))
+
+      -- Check valid scheme containing '+', '-', or '.'
+      eq(1, path_with_url([[test+abc://xyz/foo/b1]]))
+      eq(2, path_with_url([[test+abc:\\xyz\foo\b1]]))
       eq(1, path_with_url([[test-abc://xyz/foo/b3]]))
       eq(2, path_with_url([[test-abc:\\xyz\foo\b3]]))
+      eq(1, path_with_url([[test.abc://xyz/foo/b1]]))
+      eq(2, path_with_url([[test.abc:\\xyz\foo\b1]]))
+
+      -- Check valid scheme with full suite of allowed characters
+      eq(1, path_with_url([[test+abc-123.ghi://xyz/foo/b1]]))
+      eq(2, path_with_url([[test+abc-123.ghi:\\xyz\foo\b1]]))
+
+      -- Check invalid scheme starting or ending wiht '+', '-', or '.'
       eq(0, path_with_url([[-test://xyz/foo/b4]]))
       eq(0, path_with_url([[test-://xyz/foo/b5]]))
+      eq(0, path_with_url([[+test://xyz/foo/b4]]))
+      eq(0, path_with_url([[test+://xyz/foo/b5]]))
+      eq(0, path_with_url([[.test://xyz/foo/b4]]))
+      eq(0, path_with_url([[test.://xyz/foo/b5]]))
+
+      -- Check additional valid scheme containing '+', '-', or '.'
       eq(1, path_with_url([[test-C:/xyz/foo/b5]]))
       eq(1, path_with_url([[test-custom:/xyz/foo/b5]]))
+      eq(1, path_with_url([[test+C:/xyz/foo/b5]]))
+      eq(1, path_with_url([[test+custom:/xyz/foo/b5]]))
+      eq(1, path_with_url([[test.C:/xyz/foo/b5]]))
+      eq(1, path_with_url([[test.custom:/xyz/foo/b5]]))
+
+      -- Check invalid scheme representing drive letter
       eq(0, path_with_url([[c:/xyz/foo/b5]]))
       eq(0, path_with_url([[C:/xyz/foo/b5]]))
     end)