Mainline development mode

[JakeGinnivan]

This builds on top of #832

It's pretty rough but will ship this as part of 4.0.0 beta if it looks ok

[pascalberger]

The branching strategy images still contain -unstable tags. Since alpha is the default in this PR I would change this in the images too.

[pascalberger]

Isn't this set through GitVersion in the build process and should be kept to 1.0.0 in the repo?

[JakeGinnivan]

Ran build script locally, forgot to reset these changes

[pascalberger]

We should also make sure to document the new mainline mode properly. There is already a mainline documentation available which should be updated with these changes.

[stocksr]

typo: from from

[aarondandy]

I did a local build and tried it out, worked great and as expected during my quick testing.

My 🍠

assembly-versioning-scheme: MajorMinorPatch
mode: Mainline
branches:
  master:
    tag: ''
ignore:
  sha: []

I did get two test failures though when running build.ps1:
Test Count: 451, Passed: 449, Failed: 2, Inconclusive: 0, Skipped: 0

1) Error : ConfigProviderTests.CanWriteOutEffectiveConfiguration
Shouldly.ShouldAssertException : Cannot find a difftool to use, please open an issue or a PR to add support for your dif
ftool.

2) Error : DocumentationTests.ConfigurationDocumentationIsUpToDate
Shouldly.ShouldAssertException : configurationDocumentationFile
    should contain (case insensitive comparison)
"**`no-bump-message:`**"
    but was actually
"# Configuration
GitVersion 3.0 is mainly powered by configuration and no longer has branching strat..."