Add pulsar in open source editors

[mbvgua]

Add Pulsar, a hackable and community-led text-editor to the list of open source editors.

Please confirm this pull request meets the following requirements:

• I followed the contributing guidelines: https://github.com/github/explore/blob/main/CONTRIBUTING.md.
• I am not the sole author or employee of a company who created either the topic I am modifying/adding or the collection entry I am modifying/adding.

Which change are you proposing?

• Suggesting edits to an existing topic or collection
• Curating a new topic or collection
• Something that does not neatly fit into the binary options above

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Editing an existing topic or collection

I'm suggesting these edits to an existing topic or collection:

• Image (and my file is *.png, square, dimensions 288x288, size <= 75 kB)
• Content (and my changes are in index.md)

Pulsar is now a popular fork of Atom and as such its only righ tthat it be included in the list of open source editors.

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Please note: we will close your PR without comment if you do not check the boxes above and provide ALL requested information.