Add script to alphabetise submodule list

[Alhadis]

This PR adds a script to sort the contents of .gitmodules each time a new grammar is added.

Description

We see this all the time:

This happens because newly-registered submodules are always added at the very bottom of the .gitmodules file, meaning that merging one PR usually blocks the next one until the submitter has resolved conflicts manually. This slows everybody down: a user has to fix the conflicts themselves, then @lildude needs to approve the changes later.

We can remedy this simply by forcing the submodule list to be alphabetised (case-sensitively). A test has been added to ensure that manually-added modules still get ordered correctly.

As of this writing, there are currently nine PRs sitting open which have conflicting module-lists: #4030, #4005, #3983, #3869, #3802, #3772, #3764, #3689, #3720.

I haven't even bothered to count how many that are closed which have had similar conflicts. 😉

Checklist:

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

[pchaigno]

I haven't even bothered to count how many that are closed which have had similar conflicts. 😉

Who, you scared me. For a second, I read I have even 😲

[pchaigno]

Definitely a good idea!

[lildude]

I was thinking about doing this the other day after seeing another conflict. ✨ 🙇

[Alhadis]

BTW, I ran into a few issues when testing the updated add-grammar script:

1. Docker wasn't installed, so I had to sudo apt-get install docker

2. Docker still wasn't installed, so I had to double-check what the hell the docker package was, and found out the correct name is actually docker-ce (??!)

3. The correct Docker was installed, but it complained about inadequate permissions:

Warning: failed to get default registry endpoint from daemon (Got permission denied while trying to connect to the Docker daemon socket at unix:///var/run/docker.sock: Get http://%2Fvar%2Frun%2Fdocker.sock/v1.35/info: dial unix /var/run/docker.sock: connect: permission denied). Using system default: https://index.docker.io/v1/
Got permission denied while trying to connect to the Docker daemon socket at unix:///var/run/docker.sock: Post http://%2Fvar%2Frun%2Fdocker.sock/v1.35/images/create?fromImage=linguist%2Fgrammar-compiler&tag=latest: dial unix /var/run/docker.sock: connect: permission denied

This makes for some pretty unfriendly UX; should the scripts be updated to deal with these hurdles? Even bailing early in add-grammar would be preferable to bailing halfway, because each of the above three errors left the repository with half-registered grammar modules... 😞

[lildude]

This makes for some pretty unfriendly UX; should the scripts be updated to deal with these hurdles? Even bailing early in add-grammar would be preferable to bailing halfway, because each of the above three errors left the repository with half-registered grammar modules... 😞

Yeah, I thought this may have been a problem too. Wanna open an issue so we can track and not forget this?

[Alhadis]

Done. 😉 See #4056.