vscode: Support languages.configuration.onEnterRules and add OnEnterRule#previousLineText

[planger]

What it does

With this change, we read languages.configuration.onEnterRules defined via the package.json in a VS Code extension and pass them on to the language configuration for Monaco, which was previously only supported for programmatically added language configurations (#11202), and add support for the optional property OnEnterRule#previousLineText in enter rules, which was missing before (#11131).

Fixes #11202
Fixes #11131

Contributed on behalf of STMicroelectronics

How to test

The Theia example app includes the vscode.python extension, which defines the following onEnterRules in its package.json.

vscode.python defines that hitting enter after e.g. if TRUE: the next line should indent once:

  "onEnterRules": [
    {
      "beforeText": "^\\s*(?:def|class|for|if|elif|else|while|try|with|finally|except|async).*?:\\s*$",
      "action": {
        "indent": "indent"
      }
    }
  ]

Before this change, the indentation didn't work:

After this change, the indentation should work:

To test OnEnterRule#previousLineText, we can add the following rule to plugins/vscode.python/extension/language-configuration.json and restart the Theia backend:

    {
      "beforeText": "^\\s*(?:DONALD)$",
      "previousLineText": "^\\s*(?:WHOIS)$",
      "action": {
        "indent": "none",
        "appendText": "DUCK"
      }
    }

After that the previousLineText should be considered as follows:

Finally, it'd be good to very that programmatically defined enter rules are still working as expected, which can be verified e.g. with an HTML file in the Theia example app:

Review checklist

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

Reminder for reviewers

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[colin-grant-work]

The changes are correct and the functionality works as described. 👍

[planger]

Thanks a lot for the review @colin-grant-work!
@msujew do you want to look at the change too, or can we merge it?

[msujew]

@planger I took a look and everything works correctly. Code also looks good to me 👍

[planger]

Awesome, thanks @msujew!