move from appveyor into github actions build

[unitycoder]

• [x] create github actions config (build & release)
• [x] build only main branch or on "#build" message only?
• [x] give automatic version number to release (like appveyor did? or start from same counter value at least?) takes current version number from yaml : github.run_number**
• [ ] remove appveyor files and setup *or can leave as backup

[unitycoder]

also would be nice to feed that version number into build (i guess could run script to replace placeholder in some c# script, with that github.run_number value?) https://github.com/unitycoder/UnityLauncherPro/issues/130

[unitycoder]

added github actions, will look into versio number later

[unitycoder]

actually still need to fix the release infos, should display those commits at least..

now:

---

before: (appveyor)

[unitycoder]

this probably works, https://github.com/unitycoder/UnityLauncherPro/commit/2255357a7f9788c7ce6f09e40ee7fd6798f781b2#diff-7829468e86c1cc5d5133195b5cb48e1ff6c75e3e9203777f6b2e379d9e4882b3R44