Rewrite script/add-grammar as a plain shell-script

[Alhadis]

Description

I recently suggested that script/add-grammar be rewritten as a plain shell-script, since 90% of it is nothing but executing external commands. Really the only thing we need Ruby for here is normalising user-provided grammar specifiers, a function that's now handled with a new script: normalise-url:

$ script/normalise-url Alhadis/language-etc bitbucket:bitlang/sublime_cobol
https://github.com/Alhadis/language-etc.git
https://bitbucket.org/bitlang/sublime_cobol.git

I've gone and hardened the normalisation logic to catch and correct malformed URLs (when ambiguity isn't a concern), something which the current script does only a minimal job of. For example:

$ script/normalise-url --protocol=https git@GITHuB.CoM://////Alhadis/language-etc///////
https://github.com/Alhadis/language-etc.git
$ script/normalise-url git+ssh://github.com////github/linguist.git
git://github.com/github/linguist.git

Checklist:

• I am adding new or changing current functionality
  • I have added or updated the tests for the new or changed functionality.

/cc @lildude

[lildude]

A quick first pass-through with my handy linters. Mostly nits.

[lildude]

Two more non-blocking quick fixes to keep things consistent so feel free to merge the suggestions and merge.

Thanks for this rewrite 🙇