Allow disabling fast mode

Currently Fast mode is not configurable:
https://github.com/adafruit/Adafruit_CircuitPython_TLV493D/blob/2.0.12/adafruit_tlv493d.py#L128

On my unit a known issue occurs, which can be prevented by disabling fast mode - the sensor "freezes" and would have to be reset, so it seems: https://community.infineon.com/t5/3D-sensors/TLV493-A1B6-lockup-crash-bug/td-p/344863

After a variing timespan the unit always sends the same measurements until it is being reset, which is not desirable and not a good workaround.
So I've tested with fast mode disabled and found it not only prevents the issue from happening but also is still plenty fast for my purpose (which is reading on an interval of roughly 50ms).

Therefore I'd suggest that fast mode can be configured on initialization.

Start in adafruit_tlv493d.py at the fast-mode configuration around line 128 and inspect how initialization sets the sensor mode. Make fast mode configurable during initialization while preserving the current default behavior, then verify that disabling it prevents fast mode from being enabled.

Written by the indexing model from the issue text.