Rework special values syntax

[ccremer]

ModuleSync knows 2 kinds of special values:

• delete
• unmanaged

From a syntax perspective, it's easy to mistake them for variables that are being used in templates.
However, greposync shall make this more explicit and also make those free for use in templates.

Furthermore, greposync supports more special values:

• targetPath
• initOnly (possibly, see Support init files #100 )

Proposal:

• Rename parameter targetPath to :special:targetPath
• Rename parameter delete to :special:delete *
• Rename parameter unmanaged to :special:unmanaged *

*This will cause a breaking change for all .sync.yml files in all managed repos. To make migration easier, the following shall be done:

• Offer a migration CLI flag, e.g. --migrate-special-values that replaces delete and unmanaged with the new parameter names and includes the change in the commit. Note: We'd have to search-replace line-by-line and cannot read/write as YAML, as that likely removes any user comments.
• If only the legacy parameters are present, use those
• If new parameters are present, use the new ones (regardless whether legacy parameters are present or not)