Add spellchecking action

[KnutAM]

It does find a few mistakes around, see https://github.com/Ferrite-FEM/Ferrite.jl/actions/runs/7427267344/job/20212641841 (Edit: fixed in #866)
But seems somewhat fragile as I had to add quite some exceptions.
On the other hand, since it should only run on pull requests it won't indicate CI failure even if not fixed.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.25%. Comparing base (a347761) to head (2a9e0ed).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #865   +/-   ##
=======================================
  Coverage   94.25%   94.25%           
=======================================
  Files          40       40           
  Lines        6757     6757           
=======================================
  Hits         6369     6369           
  Misses        388      388           

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[termi-official]

Since typos seem to sneak in regularly I would say lets add this to the CI to catch typos early on.

[termi-official]

I think the .typos.toml is sufficiently easy to find (and remember) that we do not need any additional documentation for that.

[KnutAM]

If this gets too annoying then it can easily be removed, but this should reduce review work a bit