Clear All Decorations in dispose

[Baricus]

This PR fixes issue #600, by calling dispose for the decorationType member of InactiveRegionsFeature objects during their own disposal. Doing so removes the applied decorations, preventing them from persisting after the the extension is stopped or restarted.

[HighCommander4]

Thanks for the fix! I confirmed with a local test that this fixes #600.

It would be nice in principle to add an automated test for this, but looking at https://github.com/clangd/vscode-clangd/blob/master/test/inactive-regions.test.ts that doesn't seem straightforward to do using the existing mocks, and I feel a bit out of depth with typescript testing to suggest a way to do it that involves new mocks. If someone would like to write a test as a follow-up, please feel free :)