Upgrade to Vale 3.18.0 to support MyST, moar new features 🎉

[stevepiercy]

Vale 3.18.0 was released today and now supports MyST and I found a script to enforce one sentence per line. 🎉

• Upgrade to 3.18.0 in project requirements.
• Configure following https://docs.vale.sh/formats/myst.
• Remove spellings that were added to accept.txt and which Vale now ignores.
• Add one sentence per line script https://gist.github.com/jdkato/81508f54b34d7f0006aa90491257196b

Ref:

• Ignore MyST syntax labels vale-cli/vale#667