Raise error if `install_origins` if absent from manifest for `self-hosted` and MV3

[diox]

MV3 self-hosted add-ons will need to specify in their manifest what origins to allow installs from.

Therefore, in the linter, when self-hosted parameter is true and max-manifest-version is 3 or higher, if install_origins is missing from the manifest, an error should be issued.

Note that an empty install_origins value ([]) should be allowed for local installation of those add-ons without a web server - so we should only return an error if the key is absent entirely, and only for self-hosted if MV3 are explicitly allowed.