feat: Support fallback environment source branch

[Kieranties]

Description

Exposes simpleversion.sourcebranch - enabling a fallback environment variable for any build system to use.

The primary concern is to alleviate TeamCity issues as identified in #141 and #142

By setting the environment variable any build system can be explicit about the canonical branch name to be used in version calculation

Fixes #141
Replaces #142

Change Type

• Documentation: Changes to docs are included
• Infrastructure: Changes to build scripts/non-deliverables
• Bug Fix: Fixes an issue in implementation
• New Feature: Adds new functionality in a non-breaking manner
• Breaking Change: Implements a fix or feature in a breaking manner
• Other -

Quality Checks

• Unit tests have been added/updated
• Acceptance tests have been added/updated
• Local builds/testing are successful
• Code coverage has not decreased
• Code comments have been provided (where appropriate)
• Core styles have been followed
• Documentation has been added/updated

[ghost]

Looking great, thanks for this.

[ghost]

@Kieranties can confirm the fallback environment works. One little note - i've set the simpleversion.sourcebranch to teamcity.build.branch instead of teamcity.build.vcs.branch. because that allows for better templating and generic usage. Works both ways - the difference is just if the refs/ is added or not.