Inserting a mention should not add extra white spaces

[oleq]

Suggested merge commit message (convention)

Fix (mention): Inserting a mention should not add an extra white space if there was one already. A white space should not follow a mention inserted in brackets. Closes #4651.

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Additional information

In this PR I addressed two issues:

• The original one from Mention should not add a white space if a position already followed by one #4651
  • ( foo [] bar -> mention -> foo @mention[] bar instead of foo @mention [] bar)
• An extra one that I figured out was cheap to implement and would benefit the users
  • ([]) -> mention -> (@mention) instead of (@mention )

I think there are many improvements left related to inserting mentions in different places, for instance, I would expect "[] -> type a mention -> "@mention -> type " -> "@mention" but this one falls into text transformation category.