Add a tool to find uncovered files

[liamappelbe]

If a file is not imported by the tests, it will not be compiled, so won't be mentioned in the source report. The compiler/VM won't be aware that the file even exists. This means that entire files can be uncovered without affecting the coverage report.

We could add a tool (or an option in an existing tool) that searches for all the .dart files in the package directory and reports any that are not mentioned in the coverage data.

The tool would need some filtering options, because it's common to have scripts that are not intended to be tested or used outside of the package (eg scripts in the tool directory). Maybe by default it should only look in the src subdirectory.

The tool will also need to support workspaces when they are added.

Context: flutter/flutter#145609