Format files with dprint (#711)

The dprint formatter is a flexible system which will use sandboxed
WebAssembly formatters to format our code (mostly: it calls out to
`rustfmt` for Rust code).

A particularly interesting feature is that dprint can format Rust code
blocks in the Markdown files. However, before we turn that on, we need
to have a way to normalize the Markdown text as it is extracted[1].
That is so that the word put into the translations is kept after the
reformatting.

[1]: https://github.com/google/mdbook-i18n-helpers/issues/19

.github/workflows/build.yml

@@ -21,8 +21,8 @@ jobs:
           rustup default nightly
           rustup component add rustfmt
 
-      - name: Check Rust formatting
-        run: cargo fmt --all -- --check
+      - name: Check formatting
+        uses: dprint/check@v2.2
 
   cargo:
     runs-on: ubuntu-latest