grass.app: Add mapset create subcommand

[wenzeslaus]

For symmetry with project create, this adds a new mapset create subcommand. The mapset subcommand can be used, similarly to project, to group operations related to mapsets.

Locking will be moved under mapset in #6437, so besides the availability of the subcommand for testing of other subcommands, the move of locking is the main motivation for adding the mapset create subcommand now.

The mapset creation uses the semi-internal function grass.grassdb.create.create_mapset. The function parameters were adjusted to allow for a single path being provided (which was already supported through path resolution function, but not supported though the parameters).

There are no parameters yet, and only basic creation and creation failure are handled (no computational region, overwrite, or search path). Tests cover creation and basic expectations of the new mapset, that is being separate from the default mapset, but part of the project.

Example

python -m grass.app project create data/project --crs EPSG:3358
python -m grass.app mapset create data/project/task_1
python -m grass.app run --project data/project/task_1 g.region res=0.1

Assuming #6442:

grass project create data/project --crs EPSG:3358
grass mapset create data/project/task_1
grass run --project data/project/task_1 g.region res=0.1

Comparison with the classic CLI:

grass -c EPSG:3358 data/project -e
grass -c data/project/task_1 -e
grass data/project/task_1 --exec g.region res=0.1

Similarly to the project create subcommand, mapset create makes the create operation more clear, rather than driven by a flag (-c) as additional operation to whatever is the main thing happening, it makes distinction between project and mapset creation explicit (not relying on the combination of path and CRS), and no -e is required to avoid the interactive start. On the other hand, the three commands are always required for the above example (while the classic interface can do it in two, combining the mapset creation with the first tool run).