Syntax Highlighting regex+format strings (rf)

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* **Editor name and version**: VSCode 1.75.1
* **Platform**: macOS
* **Color scheme**: Dark+ (default)
* **MagicPython version**: 1.1.0
* **A sreenshot**:
* **5-10 lines of surrounding code**:
```python
myString = "baz"
MYCONST = "qux"
myDict = {
"foo": "bar"
}

correctColoring = rf"(?=[\w]+){myString}{MYCONST}[\d]+"

incorrectDictionary = rf"(?=[\w]+){myDict['foo']}[\d]+"
regularFString = f"word{myDict['foo']}number"
```

This is a followup to #186. After upgrading to 1.1.0, I'm very excited that rf strings support both regex style and format string. In the attached screenshot, behold how correctly varied `correctColoring` is. However, any dictionary or class inside of the {} in an rf string is not colored correctly. See how `myDict['foo']` varies between `incorrectDictionary` and `regularFString`. The square brackets should be purple, and 'foo' orange, but instead it is all absorbed into red.

Start with the rf-string examples in this issue and compare the highlighting behavior of myDict['foo'] with the regular f-string example. Review the implementation related to the follow-up in issue #186, reproduce the incorrect coloring in VSCode, and consider the issue done when dictionary or class expressions inside rf-string format fields are highlighted consistently.

Written by the indexing model from the issue text.