Add new inline Markdown underline rule

[wxwxwxwx9]

What is the purpose of this pull request?

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

This PR is related to #1473 by addressing the intermediary addition of $$ syntax for underlining, on top of the existing ++, before we fully deprecate ++ for underlining and transition to only using $$.

Overview of changes:
I reused our internal existing plugin, markdown-it-dimmed, as a dynamic markdown-it plugin that allows us to define double-delimiter inline rules (e.g. ++, $$, %%). We use $$ for underlining in this PR.

In order to introduce the logging of soon-to-be deprecated underline syntax warning for ++, I also had to pass down the current working file (context) parameter down, so that it can be passed into the render functions of markdown-it library.

Anything you'd like to highlight / discuss:
Whether the implementation for this version is fine or there should be some other tweaks.

Testing instructions:
Create a new md file in your Markbind and test out the following syntaxes: ++underline++, $$underline$$.

You should be able to mix-and-match the different syntaxes and achieve a combined result (e.g. %%++underlined_dimmed++%% should give you a text that is underlined and dimmed).

If there is a ++ syntax used for underlining, there will be warning logged as well.

Proposed commit message: (wrap lines at 72 characters)
Add new inline Markdown underline rule.

We will be transitioning from ++ to $$ for underlining syntax
in v3.0 as ++ will be used for increasing text size instead.

To ensure a smoother transition for users, let's add $$ syntax
for underline as well, so that users have time to switch their
underline syntax over to $$ before we deprecate ++ fully.

---

Checklist: ☑️

• Updated the documentation for feature additions and enhancements
• Added tests for bug fixes or features
• Linked all related issues
• No blatantly unrelated changes
• Pinged someone for a review!

[ang-zeyu]

@wxwxwxwx9 looks like there's still some failing tests, could you check these out?

[wxwxwxwx9]

@ang-zeyu thanks for highlighting! There were some minor areas that I overlooked. I have updated them, it should be fine now :-)

[ang-zeyu]

Lgtm 👍