Implement canonical formatter (#1107)

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

pkl-formatter/src/test/files/FormatterSnippetTests/output/multi-line-strings.pkl

@@ -0,0 +1,33 @@
+foo =
+  """
+  asd \(new {
+    bar = 1
+  }) asd
+  """
+
+bar =
+  """
+  line 1
+    line 2
+      line3
+  """
+
+baz =
+  """
+  \n
+  \(bar)
+  line
+  \u{123}
+  """
+
+// remove unneeded spaces
+qux =
+  """
+  foo
+
+  bar
+
+  baz
+    
+  \(foo)
+  """