Improve error message on trailing comma in method definition

The error produced by this (invalid) code is confusing and not helpful:

def run_build_script(
   workspace_dir: ARGV[0],
   output_dir: ARGV[1],
   cache_dir: ARGV[2],
   stack: ENV.fetch("STACK"),
   ruby_version: ENV.fetch("STACK"),
   io: STDOUT,
)
 end

$ exe/syntax_suggest /tmp/bad.rb
--> /tmp/bad.rb

syntax error, unexpected ':', expecting end-of-input

  1  def run_build_script(
> 2     workspace_dir: ARGV[0],
> 3     output_dir: ARGV[1],
> 4     cache_dir: ARGV[2],
> 5     stack: ENV.fetch("STACK"),
> 6     ruby_version: ENV.fetch("STACK"),
> 7     io: STDOUT,
  8  )

The problem is that there's a trailing comma. This code works (when you remove the comma after STDOUT):

$ cat /tmp/bad.rb
def run_build_script(
   workspace_dir: ARGV[0],
   output_dir: ARGV[1],
   cache_dir: ARGV[2],
   stack: ENV.fetch("STACK"),
   ruby_version: ENV.fetch("STACK"),
   io: STDOUT
)
 end
$ exe/syntax_suggest /tmp/bad.rb
Syntax OK

Fix Idea(s)

I'm not sure how to approach this. Possibly we could check for some variation of this regex https://rubular.com/r/aocG0GtGFjEiG2 as a brute force check. There may be a more elegant option.

Sidebar: I would love to see trailing commas allowed in method definitions. I think adding this behavior is an issue for positional args as you want that feedback that your airity is off. Idea: Allow it if and only if the method ends in a kwarg (instead of a positional arg). I don't know if it's been discussed or not.

Start with the reproducer through exe/syntax_suggest using the /tmp/bad.rb example, then compare its output with the version where the trailing comma is removed. Trace the error-reporting path to determine where this case can receive a clearer diagnostic; done means the invalid example reports the trailing comma rather than the confusing unexpected-colon error.