feat(ci): add a linter/formater for xml

Description

Uses Prettier + XML plugin. Take a look at https://github.com/prettier/plugin-xml for installation and usage

Results

XML will now be somewhat checked and formatted.

How to test it?

For Prettier:

• Checkout second commit
• Install prettier and prettier XML plugin:
  • On Windows:
    • install nodejs >14 + npm (normally done for cspell)
    • npm install --global prettier @prettier/plugin-xml
  • On Linux
    • install nodejs >14 + npm. To update node easily:

      sudo npm install -g n
      sudo n stable
      hash -r

    - `sudo npm install --global prettier @prettier/plugin-xml`
    

For both

• Find a badly formatted xml file (there are a lot..)
• Stage the file and launch Sheldon:
  • An error is reported if the file is not indented correctly
• Relaunch Sheldon with -f
  • The file is reformatted

Related issue(s)

Closes #71 (closed)