Template variable containing absolute path to root taskfile

[PrincessRTFM]

My particular use case for wanting to provide this value is that I would like to define a default task that will run task -t "{{.ROOT_FILE}}" --list, such that calling task without a specific task name will print the basic help for the file. There are other possibilities, such as a task that copies some user-created template taskfile into the invocation directory, which could be accomplished via cp "{{.ROOT_FILE}}" "{{.USER_WORKING_DIR}}" on linux systems. At present, no such commands are possible.

Currently, the .ROOT_DIR variable provides only the directory of the root taskfile, which can be passed to task -d "{{.ROOT_DIR}}" --list for that particular use case. However, since the CLI accepts the -t/--taskfile argument, it is possible for a non-default filename to be used, which would prevent any task commands from definitively knowing the actual file being run.

As a side note, it may also be useful to have such a variable for included task files as well, although I don't have a specific use case for such.

[pd93]

Seems pretty reasonable to me. See #1469