Label is an alternative name for task that replace it when printed in following context eg.:
- log: when given task is up to date and is skipped from execution
- log: when given task is NOT up to date (`--status` command)
- in `--summary` and `--list` commands output
This shouldn't have any behavior changes for now. This is a code
refactor that should allow us to do further improvements on how
variables are handled, specially regarding respecting the declaration
order in Taskfiles, which should make it easier for the users.
Initial work on #218
