Add support for capture groups in onEnterRules

From @siegebell in https://github.com/Microsoft/vscode/issues/3088#issuecomment-267236002

---

Before:

// comment
//     indented|

After:

// comment
//     indented
//     |

For this case, onEnterRules could be extended to support matching groups like find/replace currently does…

languages.setLanguageConfiguration(modeID, {
    onEnterRules: [
        { // insert comment on new line while preserving the indent
            beforeText: /^\s*(\/\/\s*)\S.*$/,
            action: { indentAction: IndentAction.None, appendText: '$1' }
        }
    ]
});

[hediet]

Before:

// comment
//     indented|

After:

// comment
//     indented
//     |

For me, there are two indentations in this example. The indentation of the comment and the indentation of the text within the comment. I find it strange to use two different strategies to handle these indentations (explicit indent/dedent/none actions of the onEnter rule vs regex whitespace group references in the insert text).

Is there a scenario where it makes sense to reference a regex group that matches non-whitespace text?

Also, should indentation/dedentation be supported for nested indentations (rather than just keeping the indentation as is)?

Maybe it might also be valuable to consider embedded languages in general. Imagine using a markdown codeblock in doc-comments:

/**
 * Example:
 * ```ts
 * class Foo { <<User types enter here>>
 *
 * ```
 *
 */
function bar() {}

Ideally, the outer and inner typescript rules would be combined somehow, yielding this:

/**
 * Example:
 * ```ts
 * class Foo {
 *     <<New Cursor Position>>
 *
 * ```
 *
 */
function bar() {}

Currently VS Code ignores the rules of the inner language.

This feature would not fix #66235 anyway.

I think this feature might require more discussions. Until then, it could be investigated how parts of this feature can be implemented by extensions listening on onDidChangeTextDocument and issuing edits and selection updates instead.

[jrieken]

Right, using $1 (no curly brackets) is a good fit