[lsm6ds_base] Rename C++ files to include _base suffix

Renamed the C++ implementation files to match the component folder naming:
- lsm6ds.h -> lsm6ds_base.h
- lsm6ds.cpp -> lsm6ds_base.cpp

Updated all includes:
- lsm6ds_base/lsm6ds_base.cpp: Updated self-include
- lsm6ds_i2c/lsm6ds_i2c.h: Updated to include lsm6ds_base.h
- lsm6ds_spi/lsm6ds_spi.h: Updated to include lsm6ds_base.h

Fixed test file:
- tests/lsm6ds_test.yaml: Updated sensor platforms to use lsm6ds_i2c and lsm6ds_spi

This ensures all files follow consistent naming conventions with the _base suffix.

tests/lsm6ds_test.yaml

@@ -24,7 +24,7 @@ lsm6ds_spi:
   cs_pin: 5
 
 sensor:
-  - platform: lsm6ds
+  - platform: lsm6ds_i2c
     lsm6ds_id: my_lsm6ds_i2c
     accel_x:
       name: "I2C Accel X"
@@ -33,7 +33,7 @@ sensor:
     temperature:
       name: "I2C Temp"
 
-  - platform: lsm6ds
+  - platform: lsm6ds_spi
     lsm6ds_id: my_lsm6ds_spi
     accel_x:
       name: "SPI Accel X"