Fix MarkdownUtil#monospace not correctly applying monospace formatting to strings with backticks

[ManosSef]

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Closes Issue: #3071

Description

Modifies the logic of MarkdownUtil#monospace, so that it:

• Correctly applies monospace formatting even when the input string contains backticks, and
• Does not add backslashes, since they have no effect on backticks and are actually visible in the Discord client, meaning the result string is not faithful to the input string.

Single backticks are allowed in monospace text by using double backticks as delimiters instead, and monospace text can start with backticks by inserting an extra space between the delimiter and the string, which does not appear on the client. These improvements allow the method to properly monospace text with backticks.

It is impossible to apply monospace formatting to text that contains both single and double backticks, so that case is not handled.

This should not be "out of scope" for JDA, since there is no complex parser logic involved. The current implementation is worse for text with backticks than just manually concatenating a backtick before and after the string, since it adds backslashes to the text which are shown on the client. This PR fixes the implementation enough to bring it on par with other methods in MarkdownUtil.