Requirements

• Filling out the template is required. Any pull request that does not include enough information to be reviewed in a timely manner may be closed at the maintainers' discretion.
• All new code requires tests to ensure against regressions

Description of the Change

This PR unconditionally activates packages when a deserializer for that package is called. It also adds a new way of deferring package activation through the workspaceOpeners key. Supply it an array of workspace opener filepaths/URIs that your package watches for and it will remain deactivated until one of those openers is called. This is useful when used in conjunction with deserializers, which often are created using workspace openers.

To demonstrate how a package would benefit from this PR, consider a package that listens to the opener atom://example to create a new ExampleView that has a serializer. atom://example is listed as a workspace opener in package.json. The example:open-example-view command calls atom.workspace.open('atom://example') and is listed as an activation command in package.json. This package structure is in use by packages such as Timecop and Markdown Preview.
Previously, there was no way to safely defer activation for a package that implemented serialization because it would not be activated when deserialization occurred.

• Defining the activation command would break deserialization, as the view would get added but the package was technically not activated and as such had no registered styles, commands, menus, etc.
  • With this PR, the deserializer forces the package to be activated.
• Opening the view, closing it, reloading Atom, then reopening the view through methods such as pane:reopen-closed-item would throw an EINVAL: open atom:\example error on Windows due to the workspace opener not being registered.
  • With this PR, workspaceOpeners registers a dummy opener for the opener that then activates the package and calls the real opener.

Alternate Designs

There could be another key added to deserializers stating whether or not that deserializer should activate the package.

Why Should This Be In Core?

Deserializing happens in core.

Benefits

Deserializing, openers, and deferred activation techniques can be used together.

Possible Drawbacks

Perhaps there are some deserializers that don't need the package to be activated? In which case this will slow down Atom's load time unnecessarily.

Applicable Issues

Closes #16099