Add WebExtension topic

[shaedrich]

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)

Please replace this line with an explanation of why you think these changes should be made.

Curating a new topic or collection

• I've formatted my changes as a new folder directory, named for the topic or collection as it appears in the URL on GitHub (e.g. https://github.com/topics/[NAME] or https://github.com/collections/[NAME])
• My folder contains a *.png image (if applicable) and index.md
• All required fields in my index.md conform to the Style Guide and API docs: https://github.com/github/explore/tree/main/docs

Together, "web extension", "webextension" and "browser extension" have 5k repositories

Something that does not neatly fit into the binary options above

• My suggested edits are not about an existing topic or collection, or at least not a single one
• My suggested edits are not about curating a new topic or collection, or at least not a single one
• My suggested edits conform to the Style Guide and API docs: https://github.com/github/explore/tree/main/docs

Please replace this line with an explanation of your proposed changes.

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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.

[shaedrich]

topics::webextension topic#test_0002_does not have an alias for a topic that has its own curated content [test/topics_test.rb:22]:
alias(es) web-extension already have a topic defined, please move to 'related' instead.
Expected ["web-extension"] to be empty.

Okay, this is weird 🤨 When you look at https://github.com/topics/webextension, it's displayed as uncurated (this was why I created this PR), but apparently, despite this, the topic webextension exists in this topic as it has been created 7 years ago. Is this a bug or am I missing something? When this is fixed, I suggest merging the two.

Same with vue by the way (also curated here), web-components (curated here), and yaml (curated here)