Add `--debug` (or similar) flag to output debugging metadata about the Taskfile

[andreynering]

Sometimes users need to debug something on their Taskfiles, and we could have a flag to output information that could help them:

• Computed/expanded variables
• Computed/expanded env
• Computed/expanded sources and envs (fix: log the checked files if verbose #924)

There may be some intersection between this and #764 / #884, so this will probably be an extension of --json with extra data that would be useful for debugging but not for the editor. Because of this, it may be more intelligent to implement --json before this one (--debug).