Several tweaks to the CI configuration

[DilumAluthge]

This pull request makes the following modifications to the CI configuration:

1. Replaces '1.5' with '1'. '1' will always expand to the latest stable 1.x release of Julia. Therefore, when a new 1.x release comes out, this ensures that CI will always uses the latest Julia 1.x.
2. Use the official Codecov action to submit code coverage. When using the official Codecov action, no token is necessary. This allows code coverage to be submitted from PRs made from forks.
3. Separate the CI for Julia nightly into a separate workflow file. This allows the status of CI on Julia 1.x to be tracked separately from the status of CI on Julia nightly. Therefore, if CI fails on Julia nightly, the CI badge in the README will still say "passing". However, it is still very easy to see, at a glance, that CI is failing on Julia nightly. This is more robust than the current approach on master, which uses continue-on-error: ${{ matrix.version == 'nightly' }}. When using continue-on-error, it is very easy to miss failures on nightly.

[codecov[bot]]

Codecov Report

Merging #101 (4972df7) into master (b97cbd9) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #101   +/-   ##
=======================================
  Coverage   76.86%   76.86%           
=======================================
  Files          22       22           
  Lines        1902     1902           
=======================================
  Hits         1462     1462           
  Misses        440      440           

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update b97cbd9...4972df7. Read the comment docs.

[Jutho]

Thanks; I will probably copy this to all my repositories.