Add optional leading whitespace highlighting

[AgentHagu]

What is the purpose of this pull request?

• Documentation update
• Bug fix
• Feature addition or enhancement
• Code maintenance
• DevOps
• Improve developer experience
• Others, please explain:

Overview of changes:
This PR closes #2573. In summary, it allows users to now optionally highlight leading whitespace, giving them more options when using MarkBind.

Users can now add an optional + after their highlight index specifier to indicate that the index should be inclusive of leading whitespace. For example, previously 1[:4] would highlight the abcd in abcd. But now, by using 1[:4]+, MarkBind will highlight ab in abcd (i.e. it will also highlight the first 2 spaces in the string).

Anything you'd like to highlight/discuss:
nil

Testing instructions:
A test similar to the example brought up in #2573 can be used to test the new functionality. For example:

```{highlight-lines="1[:4]"}
  abcd
```

abcd should be highlighted, i.e. exclusive of the 2 whitespaces in the beginning.

```{highlight-lines="1[:4]+"}
  abcd
```

Now, ab should be highlighted, since the "+" symbol was added, so leading whitespaces are included in the bounds calculation.

Proposed commit message: (wrap lines at 72 characters)
Add optional leading whitespace highlighting

Currently, there is no way to include highlighting of leading
whitespace when using the highlight-lines attribute. Providing such
an option may be beneficial to users, giving them more freedom
when using MarkBind.

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Checklist: ☑️

• Updated the documentation for feature additions and enhancements
• Added tests for bug fixes or features
• Linked all related issues
• No unrelated changes

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Reviewer checklist:

Indicate the SEMVER impact of the PR:

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• Minor (when you add functionality in a backward compatible manner)
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Breaking change release note preparation (if applicable):

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Give a brief explanation note about:

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