Highlighting may disappear when switching between tabs

[m-flak]

Issue

When switching between tabs, the highlighting line decorations (not the squiggles) may disappear and will not reappear unless you reinvoke Show Code Coverage.

I'm working in a very, very large Angular codebase. Project size could have something do with this. Or more likely, VSCode text editor's line decorations aren't meant to be persistent like diagnostics.

Reproduction Steps

1. Invoke the Show Code Coverage command
2. Switch between documents
3. The highlighting will be gone until the Show Code Coverage command is invoked again.

Proposed Solution

The highlighting could be put into a separate, toggleable command to address #102.

Given the possibility that VSCode text editor's line decorations aren't meant to be persistent like diagnostics, I would suggest moving the highlighting logic into its own Highlight Uncovered Lines command.