Get parse errors and warnings directly from the Monaco editor.

[joshpoll]

Parse errors currently come from the jsonc parser, and we don't have access to parse warnings for trailing commas or comments so they don't show up in the log.

This requires us to first implement https://github.com/react-monaco-editor/react-monaco-editor/issues/501, which will give us access to error and warning information from the Monaco editor.

[domoritz]

Similarly, the schema validation could come from the editor instead of ajv (whose error messages are hard to read).