Add tests for #46 and include automated github action testing

[lil5]

I have not changed anything in the src code, except for some linting.

This PR includes:

• A Github Action that with run a test on each PR, to help against regression
• A couple of tests covering the code written in #26
• Some linting
• Ignore coverage directory flutter test --coverage

[flutter-painter]

@lil5 out of curiosity can anyone run the github action in .github/workflows/test.yml ? Excuse my ignorance but what is the command ?

[lil5]

@flutter-painter I've had to merge tests into my master branch to test the CI.

All looks fine: https://github.com/lil5/csv/actions/runs/1484508418

If you want to test the CI yourself without merging first you can look into using https://github.com/nektos/act

I've added this commit to allow manual runs see docs how to run CI's manually.