Implement canonical formatter (#1107)

CLI commands also added: `pkl format check` and `pkl format apply`.

pkl-formatter/src/test/files/FormatterSnippetTests/input/when.pkl

@@ -0,0 +1,9 @@
+foo {
+  when  (true)
+  {
+  bar = 1
+  }
+  else {
+    bar = 2
+  }
+}