extensions: show extensions decorations in separate columns as experimental feature

Created by: taras-yemets

First iteration on https://github.com/sourcegraph/sourcegraph/issues/17434

Design

When experimentalFeatures.enableExtensionsDecorationsColumnView setting is enabled:

• extension decorations supporting the column view (as for now, git-extras is the only extension supporting the column view: https://github.com/sourcegraph/sourcegraph-git-extras/pull/276) will be rendered in a separate column on the blob page
• extensions decorations are not rendered in the new reference panel blob view
• extension decorations for the diff view and browser extension (native integrations) remain unchanged.

Context: we're investigating the possibility of showing extensions decorations in separate columns (Figma). This PR is the first iteration touching only git-extras extension. We're going to dogfood it internally, then, depending on the feedback, enhance git blame and proceed with other extensions.

Should be merged with https://github.com/sourcegraph/sourcegraph-git-extras/pull/276

	Experimental feature disabled	Experimental feature enabled
Git-extras		hover on blame decoration hover on blame decoration when line is selected keyboard navigation (Tab, Shift+Tab) when line is selected
Git-extras + codecov		with tooltip

Test plan

• sg run web-standalone
• (optional) sideload git-extras extension with https://github.com/sourcegraph/sourcegraph-git-extras/pull/276 revision to have only two blame states: show for the whole file and off (if sideloaded edit extensionID to be git-extras)
• ensure git blame on the blob page is displayed in a separate column
• ensure the file diff view and the browser extension blame display remain unchanged

App preview:

• Web
• Storybook

Check out the client app preview documentation to learn more.