Add checker librsvg

[naomiceron]

This draft will solve #1494
I've been having problems making the test pass, but I will leave this draft PR in case it can help anybody.
If you have any notes, please let me know.

[terriko]

It looks like you'll need to rebase due to the big changes that have happened in the readme and manual docs. If the git conflicts look too complicated, you can grab the current README.md and doc/MANUAL.md from origin and re-apply the format_checkers changes.

Usually before I do something like this I save a copy of what I had into a tmp directory, just in case so the whole thing would look something like this:

Save a copy of those two files that we're going to revert:

mkdir ~/tmp
cp README.md ~/tmp/
cp doc/MANUAL.md ~/tmp/

Revert those two files:

git checkout -f README.md
git checkout -f doc/MANUAL.md

Rebase the whole tree:

git pull --rebase origin master

Then re-run format checkers:

python cve_bin_tool/format_checkers.py

[naomiceron]

Thank you! it looks like it didn't bring me the files correctly, so I will look into it.

[terriko]

Ah, git checkout -f may be getting the copy of the file that lives in this PR. You'll want the one from main/origin. A quick google search says the commands you probably want are as follows:

git fetch origin/main
git checkout origin/main -- README.md
git checkout origin/main -- doc/MANUAL.md

[terriko]

We've got another librsvg checker merged so I'm going to close this, but do feel free to add your tests in a separate PR if you want -- they're different than the ones we merged.