Enable vale for prose linting (#3)

* Enable vale in CI

* Add vale config

* Tweak vale

* Update config

* Update .gitignore

* Switch back

* Rename CI

* Add badges

* Tidy up CI

* Change image

* Configure vale

* Delint

* Update README

.github/styles/Google/HeadingPunctuation.yml

@@ -0,0 +1,13 @@
+extends: existence
+message: "Don't put a period at the end of a heading."
+link: "https://developers.google.com/style/capitalization#capitalization-in-titles-and-headings"
+nonword: true
+level: warning
+scope: heading
+action:
+  name: edit
+  params:
+    - trim_right
+    - "."
+tokens:
+  - '[a-z0-9][.]\s*$'