Add Pyrexp to Regular Expressions

[cyrilbois]

What does this PR do?

Add resource

For resources

Description

Pyrexp is a regex tester and visualizer

Why is this valuable (or not)?

It allows for graphically visualizing the patterns.

How do we know it's really free?

It is a web ressource, no registration

Checklist:

• [x] Read our contributing guidelines.
• [x] Search for duplicates.
• [x] Include author(s) and platform where appropriate.
• [x] Put lists in alphabetical order, correct spacing.
• [x] Add needed indications (PDF, access notes, under construction).
• [x] Used an informative name for this pull request.

Follow-up

• Check the status of GitHub Actions and resolve any reported warnings!