Add support for Cloud Firestore Security Rules

[Alhadis]

This PR adds Cloud Firestore Security Rules (hereafter just "Firestore Rules") to the list of recognised languages on GitHub.com.

Description

A user unknowingly brought the language to my attention when he asked about adding Firebase's icon to firebase.rules files. Did some investigation, and found ~2,091 search results for files named firebase.rules. I'm not sure if there are other names we should concern ourselves with. The contributor who brought this format to my attention said:

afaik it could be any *.rules file, but the de facto standard is firestore.rules (at least for firestore). I haven't seen *.rule yet

Checklist:

• I am adding a new language.
• The extension of the new language is used in hundreds of repositories on GitHub.com.
  • Search results for each extension (~2,091 results)
  • Downloaded samples were almost completely Firestore Rules
• I have included a real-world usage sample for all extensions added in this PR:
  • Sample firestore.rules taken from YeFFreY/cheesecake-frontend, licensed under MIT.
• I have included a syntax highlighting grammar.
  • jaysquared/atom-firestore-grammar
  • License: MIT
• I have included a change to the heuristics to distinguish my language from others using the same extension. None needed, I don't think.

[Alhadis]

Also, think I solved that issue we've been having:

λ linguist (cfssr): script/add-grammar https://github.com/jaysquared/atom-firestore-grammar
Checking docker is installed and running
$ docker ps
Registering new submodule: vendor/grammars/atom-firestore-grammar
$ git submodule add -f https://github.com/jaysquared/atom-firestore-grammar vendor/grammars/atom-firestore-grammar
$ script/grammar-compiler add vendor/grammars/atom-firestore-grammar
Confirming license
$ script/licensed --module /home/alhadis/Mirrors/linguist/vendor/grammars/atom-firestore-grammar
Updating grammar documentation in vendor/README.md
$ bundle exec rake samples
$ script/sort-submodules
$ script/list-grammars
  > script/list-grammars:81:in `block in to_markdown': undefined method `chomp' for nil:NilClass (NoMethodError)
  > 	from script/list-grammars:65:in `each'
  > 	from script/list-grammars:65:in `to_markdown'
  > 	from script/list-grammars:96:in `update_readme'
  > 	from script/list-grammars:105:in `<main>'
Command failed. Aborting.
λ linguist (cfssr): 

Since adding this, it hasn't had a single complaint about chomping anything I tell it to eat. =)

[pchaigno]

My initial reaction: What?! @Alhadis didn't write the grammar for that language? Oh wait, he did post a pull request with a full rewrite of the grammar...

[Alhadis]

😀Haha! Yeah, now we just need to wait for the maintainer to approve it. 😉

Going by his recent activity, I'm wondering if I shouldn't try bugging him by e-mail... there's another user's PR that's waiting to be merged too...

[Alhadis]

Oh, and about this...

  ace_mode: less
  codemirror_mode: css
  codemirror_mime_type: text/css

The highlighting applied by CodeMirror and Ace's CSS modes looks weirdly appropriate when applied to Firestore Rules, which is why I chose that over simply defaulting to plain-text. =)

[lildude]

LGTM

[pchaigno]

Going by his recent activity, I'm wondering if I shouldn't try bugging him by e-mail... there's another user's PR that's waiting to be merged too...

I'd try pinging him via GitHub.com (i.e., @hisusername) in a few days if he hasn't answered. That worked well for me in the past; I think many users receive an email notification when they're pinged. I try to keep cold-emails as a last resort.

[Alhadis]

Just so we're all clear, if you ever see me silent for over a month, it legitimately means I'm dead.