Add .sl to the list of silentlyIgnoredDirs (#14206)

In [Sapling SCM](https://sapling-scm.com/), `.sl/` is the folder where it stores its state,
analogous to `.git/` in Git. It should be ignored in Prettier like the other SCM folders.

src/cli/expand-patterns.js

@@ -46,7 +46,7 @@ async function* expandPatterns(context) {
  */
 async function* expandPatternsInternal(context) {
   // Ignores files in version control systems directories and `node_modules`
-  const silentlyIgnoredDirs = [".git", ".svn", ".hg"];
+  const silentlyIgnoredDirs = [".git", ".sl", ".svn", ".hg"];
   if (context.argv.withNodeModules !== true) {
     silentlyIgnoredDirs.push("node_modules");
   }