Improve editing experience of footnotes

What does this MR do and why?

Context: I recommend to read the Content Editor development guidelines to obtain contextual knowledge of this domain.

This MR improves the editing experience of footnotes in the Content Editor. It does the following:

• It preserves the footnote identifier when converting from Content Editor to Markdown
• It displays the footnote identifier as a read-only element.

What doesn’t this MR do?

• It doesn’t allow you to edit a footnote identifier yet.
• It doesn’t allow you to create footnotes.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Edit a Wiki page in your local environment
2. Enter the following snippet in the Classic Markdown Editor

     A footnote reference tag looks like this: [^1]
   
    This reference tag is a mix of letters and numbers. [^footnote]
   
    [^1]: This is the text inside a footnote.
    [^footnote]: This is another footnote.

3. You should notice that footnote identifiers that are not numbers are displayed correctly.
4. When switching between source and rich text modes, the footnote identifiers are preserved.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #343678 (closed)