Port v4 automatic versioning to v3 to avoid manual version update in *.yaml and isar.dart

[mrclauss]

Is your feature request related to a problem? Please describe. Currently the version on v3 branch needs to be updated manually

Describe the solution you'd like Port the solution from v4 to v3

Describe alternatives you've considered A clear and concise description of any alternative solutions or features you've considered.

Version

• Platform: iOS, Android, Mac, Windows, Linux, Web
• Flutter version: [e.g. 1.5.4]
• Isar version: [e.g. 0.5.0]

[mrclauss]

... not to forget the documentation pages...