[WIP] Add handling possible short mentions tags to ExpensiMark

[Kicu]

This is WIP

Details

The purpose of this PR is to add handling of (possible) short mention tags to ExpensiMark. The PR is work in progress but already handling most cases, but we need further discussions to finish this.

why "possible" short mentions

The only way for us to actually render short-mentions inside live-markdown (and other code using expensimark) we need to have a list of all possible mentions.
This list is not and should not be shared in any way to ExpensiMark, as this is just a parser. However in order to try to match a short mention to a list of actual users, we need some kind of output from ExpensiMark from parsing.

In this PR we define "short-mention" as anything after @ character that would be a valid email prefix.
However this creates multiple edge cases, because text that in the past would be considered just normal text, might now be a short mention.

Current edge cases (based on tests)

• test@here@gmail.com
  • prev return: "test@<a href=\"mailto:here@gmail.com\">here@gmail.com</a>"
  • now returns: "test@here<mention-short>@gmail.com</mention-short>"
• test@gmail.com@gmail.com
  • prev: <a href=\"mailto:test@gmail.com\">test@gmail.com</a>@gmail.com
  • now: <a href=\"mailto:test@gmail.com\">test@gmail.com</a><mention-short>@gmail.com</mention-short>

Some other cases can be seen in tests.

CC @puneetlath @tomekzaw @BartoszGrajdek

Fixed Issues

$ Expensify/App#38025

Tests

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