Just define a ":" builtin

It's *less code* to define this as a builtin, and it's not going
anywhere. Plus it makes fish just a little more usable without share/config.fish.

share/config.fish

@@ -106,15 +106,6 @@ else if not contains -- $__fish_data_dir/completions $fish_complete_path
     set -a fish_complete_path $__fish_data_dir/completions
 end
 
-# This cannot be in an autoload-file because `:.fish` is an invalid filename on windows.
-function : -d "no-op function"
-    # for compatibility with sh, bash, and others.
-    # Often used to insert a comment into a chain of commands without having
-    # it eat up the remainder of the line, handy in Makefiles.
-    # This command always succeeds
-    true
-end
-
 # Add a handler for when fish_user_path changes, so we can apply the same changes to PATH
 function __fish_reconstruct_path -d "Update PATH when fish_user_paths changes" --on-variable fish_user_paths
     set -l local_path $PATH

src/builtin.cpp

@@ -342,6 +342,7 @@ int builtin_false(parser_t &parser, io_streams_t &streams, wchar_t **argv) {
 // Functions that are bound to builtin_generic are handled directly by the parser.
 // NOTE: These must be kept in sorted order!
 static const builtin_data_t builtin_datas[] = {
+    {L":", &builtin_true, N_(L"Return a successful result")},
     {L"[", &builtin_test, N_(L"Test a condition")},
     {L"and", &builtin_generic, N_(L"Execute command if previous command succeeded")},
     {L"argparse", &builtin_argparse, N_(L"Parse options in fish script")},