Fix programmatic save for custom text editors

[msujew]

What it does

Closes https://github.com/eclipse-theia/theia/issues/9230

Instead of using the EditorManager (that seemingly doesn't keep track of CustomEditorWidget instances), we simply use the monaco model service to save the underlying model.

Also removes an unused RPC method $tryCloseDocument.

How to test

Follow the instructions in https://github.com/eclipse-theia/theia/issues/9230 and confirm that manual saving works as expected. Alternatively, the extension in https://github.com/eclipse-theia/theia/issues/12979 should now also work as expected.

Review checklist

• [x] As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines