Updated webextension json based on mdn

[Heck-R]

Updated webextension json based on MDN

• There were numerous property additions since the last update of the schema
• There was no proper reference for experiment_apis (maybe the "experiment" failed), so it was removed
• The descriptions are now a lot longer (mostly copied from mdn), because the shorter ones were good as topic markers, but were far from enough to aid in writing the json by themselves.
• There were some other minor additions to be more compliant to the CONTRIBUTING.md. (E.g.: titles)
• MDN examples were added as tests

[github-actions[bot]]

Thanks for the PR!

This section of the codebase is owned by @madskristensen and @hyperupcall - if they write a comment saying "LGTM" then it will be merged.

[madskristensen]

Thanks