Story: Match Git extension decoration for paths tracked by DVC

[mattseddon]

This issue is a follow up from this comment on #186:

In the Explorer we want to do our best to copy the decoration feature that the Git extension provides. This will mean that we want to:

• Un-highlight (currently greyed out) paths that are Git ignored if they are tracked by DVC (see Spike decoration provider #206).
• Highlight and add a M to paths that are tracked by DVC which have been modified since the last commit.
• Highlight and add a U to paths that are neither tracked by Git nor DVC.
• Highlight and add a D to paths that have been deleted.
• Re-grey out paths that become untracked (dvc remove) by DVC.
• Do not squash VS Code's problem matcher which will highlight files in red when a problem is found in the file.
• Improve efficiency of extension fetching list of tracked paths (currently initial load is too slow).

We can tie into the custom colors that are used by the Git extension (see list of gitDecoration settings in linked comment).
This story will be updated as we get further into the process.

[mattseddon]

replaced by #207, #318