chore(ci): add check for formatting in CI

This also adds in a .git-blame-ignore-revs file for the commit
that formats everything.

Author: ckipp01

.git-blame-ignore-revs

@@ -0,0 +1,2 @@
+# Format the codebase for the first time
+b049841d5707d5bd87be516d8cda7be2a7585eae

.github/workflows/build.yml

@@ -49,6 +49,20 @@ jobs:
 
       - run: ./mill -i -k __.mimaReportBinaryIssues
 
+  check-formatting:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+        with:
+          fetch-depth: 0
+
+      - uses: actions/setup-java@v3
+        with:
+          distribution: 'temurin'
+          java-version: 8
+
+      - run: ./mill -i -k __.checkFormat
+
   publish-sonatype:
     if: github.repository == 'com-lihaoyi/os-lib' && contains(github.ref, 'refs/tags/')
     needs: test

build.sc

@@ -6,6 +6,7 @@ import $ivy.`com.github.lolgab::mill-mima::0.0.13`
 import mill._
 import mill.define.{Task, Target}
 import mill.scalalib._
+import mill.scalalib.scalafmt._
 import mill.scalanativelib._
 import mill.scalalib.publish._
 import mill.scalalib.api.ZincWorkerUtil
@@ -124,7 +125,8 @@ trait SafeDeps extends ScalaModule {
   }
 }
 
-trait OsLibModule extends CrossScalaModule with PublishModule with AcyclicModule with SafeDeps {
+trait OsLibModule extends CrossScalaModule with PublishModule with AcyclicModule with SafeDeps
+    with ScalafmtModule {
   def publishVersion = VcsVersion.vcsState().format()
   def pomSettings = PomSettings(
     description = artifactName(),