feat: accept a fallback value in all formatted expressions

[piedar]

Description

This PR extends the custom formatting syntax to support a fallback value in a variable expression, to be consistent with the fallback syntax for environment variables. It also adds the ability to specify a more complex fallback value in a quoted string.

For example

assembly-file-versioning-format: '{Major}.{Minor}.{Patch}.{WeightedPreReleaseNumber ?? 0}'
assembly-informational-format: '{AssemblySemFileVer}+{env:BUILD_TAG ?? "fancy-string*with%special#characters"}'

Motivation and Context

I tried to use

assembly-file-versioning-format: '{Major}.{Minor}.{Patch}.{WeightedPreReleaseNumber}'

but that fails for versions where there is no pre-release label because WeightedPreReleaseNumber is null. So then I tried

assembly-file-versioning-format: '{Major}.{Minor}.{Patch}.{WeightedPreReleaseNumber ?? 0}'

but discovered that this syntax was only supported for environment variables.

How Has This Been Tested?

• added unit tests to better cover existing cases
• added unit tests to cover new cases
• examined references to discover that the changed code is used only for custom version formatting

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[asbjornu]

This is great!

[asbjornu]

Thank you so much for improving GitVersion, @piedar! 🙏

[github-actions]

🎉 This issue has been resolved in version 5.2.4 🎉
The release is available on:

• GitHub Release
• Chocolatey Package
• .Net Global Tool

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