Add code coverage metrics using Coverlet and Codecov about luhncheck

[MylesFTOP]

Now added and reporting via Codecov - current coverage for LuhnLibrary is 88.9%, with only one method uncovered (CheckLuhnSuffixReturnString, which is essentially a wrapper for CheckLuhnSuffixReturnBool to return strings of "valid" or "not valid" instead of a boolean).

[MylesFTOP]

Added to Github Action with commits 7c73567. Coverage has dropped from 89% to 22%, as command-line instruction to exclude LuhnLibrary (which is only UI logic) does not appear to have processed.

[MylesFTOP]

Commit e201397 added /p:CollectCoverage=true to dotnet test, which shows that the exclude parameter isn't being picked up from the command line (despite it both having had versions with and without escaped quotes). Need to look into this further.

[MylesFTOP]

Anecdotally, I have noticed that the exclude/include parameters don't seem to pick up from the runsettings whenever I've used it on local builds (which is why I still kick it off from the command line). Will try adding it to the runsettings now to see if it works from Github.

[MylesFTOP]

Exclude is now working on the command line using unescaped quotes (showing 90% coverage, as lines containing just method names and braces appear to have been excluded from the latest run - compare before and after).

Identical exclude is present in coverlet.runsettings, so will remove the exclude from the command line.

[MylesFTOP]

Removed command line exclude in ac44a9b, but even though the Actions output and coverage report both show that LuhnCheck was included in the analysis (see previous coverage report that did not analyse LuhnCheck when /p:Exclude="[LuhnCheck]*" was on the command line), the Codecov statistic still shows 90% coverage (i.e. it is only taking LuhnLibrary into account as specified).

It seems that if the Exclude parameter is passed in at the command line, it does not analyse any assemblies specified, but if it is passed in from the runsettings, it will analyse those assemblies but ignore them from the final statistic.