Fix issue where twitter/x usernames with _ would result in null

[harsimran-d]

What does this PR do?

update the regex for extractTwitterUsername to include '_'

Where should the reviewer start?

start in src/features/talent/utils/extractUsername.ts

How should this be manually tested?

as described in the linked issue #742

Any background context you want to provide?

twitter/x usernames do allow '_'
My own username of harsimran_d was resulting in null although its perfectly valid

What are the relevant issues?

Closes #742

Screenshots (if appropriate)

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