feat(markdown): add rumdl server and fixer

[vEnhance]

Implements #5064. Some notes:

• I basically copied feat(markdown): add marksman lsp #4565 and Add markdownlint fixer for markdown #5066 for the linter and fixer respectively.
• I have configuration options for both the linter and the fixer separately, but I could make them a single option if desired. Although in practice I expect the options list to usually be blank.
• For finding the project root, I look for both .rumdl.toml or rumdl.toml and take the shorter one if both are found. Otherwise it looks for pyproject.toml, else .git.
  • I assume it's overkill for ALE to read pyproject.toml looking to see if it has a tool.rumdl section.
• If no .git or rumdl.toml/.rumdl.toml is found I just set project root as the folder of the buffer.
• Besides the two vader files, I've put the linter and fixer on my own computer in dotfiles (here's linter and fixer) to make sure they pass sanity check.

I hope this is all correct, but it'd be good if someone competent looked over this given I'm first-time contributing and had assistance from an LLM.

[vEnhance]

so sorry i apparently only did run-tests on the vader files, i should read ale-development more carefully.

./run-tests --fast -q should pass now on a8212ef

[hsanson]

Thanks, looks good to merge.