Add Taplo formatting for TOML files (#809)

See linebender/xilem#784 and
https://xi.zulipchat.com/#narrow/channel/419691-linebender/topic/Taplo.3A.20TOML.20formatting

.github/workflows/ci.yml

@@ -74,6 +74,14 @@ jobs:
       - name: cargo fmt
         run: cargo fmt --all --check
 
+      - name: Install Taplo
+        uses: uncenter/setup-taplo@09968a8ae38d66ddd3d23802c44bf6122d7aa991 # v1
+        with:
+          version: "0.9.3"
+
+      - name: Run taplo fmt
+        run: taplo fmt --check --diff
+
       - name: install ripgrep
         run: |
           sudo apt update

.taplo.toml

@@ -0,0 +1,11 @@
+# See https://taplo.tamasfe.dev/configuration/file.html
+# and https://taplo.tamasfe.dev/configuration/formatter-options.html
+
+[formatting]
+# Aligning comments with the largest line creates
+# diff noise when neighboring lines are changed.
+align_comments = false
+
+# Matches how rustfmt formats Rust code
+column_width = 100
+indent_string = "    "