Add close method to Plugin (#4670)

[gvatn]

Went for a "good first issue": #4670
Added a close() method to Plugin.
Looked at File.close and added the same implementation: Adds a readonly rid, and runs close from ops/resources.ts, returning void.

I don't think there is testing of plugins set up in cli/js/tests, but a few simple tests in test_plugin which can be checked by cargo test.

In test_plugin/test.js I record the open resources at the beginning of the file, and check for equality after running Plugin.close() in the end.
This is a simple adaptation of assertResources() in cli/js/testing.js, which would probably be preferable if plugin tests were added to cli/js/tests.

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[bartlomieju]

LGTM, thanks @gvatn