scalar: enable built-in FSMonitor on `register`

scalar: enable built-in FSMonitor on `register`

Using the built-in FSMonitor makes many common commands quite a bit
faster. So let's teach the `scalar register` command to enable the
built-in FSMonitor and kick-start the fsmonitor--daemon process (for
convenience).

For simplicity, we only support the built-in FSMonitor (and no external
file system monitor such as e.g. Watchman).

Helped-by: Junio C Hamano <redacted>
Helped-by: Derrick Stolee <redacted>
Signed-off-by: Matthew John Cheetham <redacted>
Signed-off-by: Johannes Schindelin <redacted>
Signed-off-by: Victoria Dye <redacted>
Signed-off-by: Junio C Hamano <redacted>