ci: add spellcheck

[SethFalco]

Add spellchecking to CI. This is mostly for documentation, but we may enforce it on most if not all files anyway.

Rational is that we've let typos onto the site on too many occasions, which just isn't a great look imo, so this will hopefully reduce that.

While this PR is in a draft state, it is configured to check all files. However, before merging we will enable incremental_files_only so that it only checks modified/added files.

Bug Fixes

• In _collections, replaced indetermined with indeterminate. There is no indetermined pseudo-class. This was a typo of indeterminate, which is a pseudo-class.

Chores

• Replace some instances of British English with American English.

Related

Would've prevented all of the following:

• Fix typos #2087
• Fix typos #1996
• Fix "purly" typo #1994
• Fix several typos #1868
• Correct typo in comment in removeDimensions plugin #1633