Add pre-commit routines for prettier formatting on Xacro, launch and package XML files

[jjaime2]

Description

This PR adds a pre-commit routine for prettier formatting on Xacro, launch and package XML files. Resolves #56

You must run npm install --save-dev prettier @prettier/plugin-xml prior to pre-commit run --all-files to run prettier with the XML plugin.

Note to reviewers: You don't need to read all the thousands of changed lines, just skim through and notice that our Xacro is actually readable now :)

Testing procedure

This PR does not introduce any fundamental changes to the behavior of the code, it just updates formatting

Before opening a pull request

• pre-commit run --all-files
• Run your code through pylint. pylint --recursive=y --rcfile=.pylintrc .. All warnings but fixme must be addressed.

Before Merging

• Squash & Merge

[yewon-lee]

Looks good, just left one comment for .pre-commit-config.yaml.

[sriramk117]

PR looks good to me!

I addressed the requested change, just dismissing the request because it didn't do it automatically when I pushed the commit for some reason