[Trosper3]

Adds GitHub Actions so SPADE is built and tested on every PR, and released to NuGet by pushing a version tag.

Features

• CI (ci.yml): every PR and every push to main builds the library and runs the tests (once the test project from fix(collections): hash sets independently of element order #11 is in).
• Publishing (publish.yml): pushing a tag like v0.2.0 checks that the tag matches <Version> in DiscreteParser.csproj, runs the tests, packs the library and pushes it to nuget.org.
• No stored API key: publishing uses nuget.org trusted publishing. The workflow trades a GitHub token for a short-lived NuGet key, under a policy on nuget.org that only accepts ReduxISU/SPADE running publish.yml.

Notes

• Merging this first means fix(collections): hash sets independently of element order #11, fix(grammar): accept set edges in unorderedcross again #13, fix(grammar): make is int, is set and is list check their type #15, feat(parser): read instance strings in one pass, with clear errors #16 and feat(collections): add multisets #17 get CI runs.
• The publish path can only be fully tested by the first real release tag.