Use yarn instead of npm in tidy

Author: lolbinarycat

REUSE.toml

@@ -38,7 +38,7 @@ path = [
     "triagebot.toml",
     "typos.toml",
     "package.json",
-    "package-lock.json",
+    "yarn.lock",
     "x",
     "x.ps1",
     "x.py",

bootstrap.example.toml

@@ -315,12 +315,12 @@
 # target when running tests, otherwise this can be omitted.
 #build.nodejs = "node"
 
-# The npm executable to use. Note that this is used for rustdoc-gui tests,
-# otherwise this can be omitted.
+# The yarn executable to use. Note that this is used for rustdoc-gui tests and
+# tidy js extra-checks, otherwise this can be omitted.
 #
-# Under Windows this should be `npm.cmd` or path to it (verified on nodejs v18.06), or
+# Under Windows this should be `yarn.cmd` or path to it (verified on nodejs v18.06), or
 # error will be emitted.
-#build.npm = "npm"
+#build.yarn = "yarn"
 
 # Python interpreter to use for various tasks throughout the build, notably
 # rustdoc tests, and some dist bits and pieces.