feat: add no-space-in-emphasis rule

[Pixel998]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request?

This PR implements the no-space-in-emphasis rule to disallow spaces around emphasis markers.

What changes did you make? (Give an overview)

Implemented the no-space-in-emphasis rule, along with documentation and tests.

Related Issues

Fixes #378

Is there anything you'd like reviewers to focus on?

[snitin315]

@Pixel998 Make sure to mark it as ready for review once done, the team doesn't pay attention to draft PRs.

[snitin315]

@Pixel998 Are you still working on this?

[snitin315]

We have false negatives in case of nested emphasis:

<!-- eslint markdown/no-space-in-emphasis: "error" -->

_ **bold** _ // no error

This is * italic with __ bold __ markdown * // No error reported

This is *italic with __ bold __ markdown* // Reports error for bold emphasis as expected

[snitin315]

Following is an edge case, I'm not sure if we want to solve for it in the initial version.

**** bold **** // No error,