Porting Bitbucket module to FAKE 5

[yazeedobaid]

Description

This PR ports the Bitbucket pipeline module to FAKE 5. The module contains helper methods to get Bitbucket pipeline environment variables which are set by pipeline during a build.

TODO

• New (API-)documentation for new features exist (Note: API-docs are enough, additional docs are in help/markdown)
• unit or integration test exists (or short reasoning why it doesn't make sense)
• boy scout rule: "leave the code behind in a better state than you found it" (fix warnings, obsolete members or code-style in the places you worked in)
• (if new module) the module has been linked from the "Modules" menu, edit help/templates/template.cshtml, linking to the API-reference is fine.
• (if new module) the module is in the correct namespace
• (if new module) the module is added to Fake.sln (dotnet sln Fake.sln add src/app/Fake.*/Fake.*.fsproj)
• Fake 5 API guideline is honored

[matthid]

Thanks :)