CI/CD consolidation/enhancements: macOS .app bundling

[iceiix]

Follow-on to #351, which adds GitHub Actions, to complement the existing Azure Pipelines (#346), SourceHut (#176), and AppVeyor (#9) (also considered: CircleCI #174, Travis-CI #173). Consolidate all the CI systems into one, probably GitHub Actions, with a superset of the functionality:

• cargo clippy, for reference this is executed in Azure Pipelines (move to GitHub Actions, probably fine as the same job as the build)
• debug build without stripped symbols
• Test on specific version of Rust (minimum supported stable 1.44.1)
• simplify filenames, Stevenarella.app, .exe, . (CI enhancements: app bundling, build wasm #486)
• macOS .app bundling (CI enhancements: app bundling, build wasm #486)

see notes in #351 (comment)

[MCOfficer]

Notes:

• for MacOS app bundling, here's an example: Mac bundling EndlessSkyCommunity/ESLauncher2#42
• for clippy, there's rust-clippy-check. Even more interesting, the WIP clippy action.

PS: Jesus, that's a lot of CIs :D