chore: rename targets and add ENABLE_TESTING option

Rename library targets from celrs_* to cel* (celcrsf, celserial,
cellogger, cellog) and add cel:: namespace aliases. Add cel::cel
umbrella target that links all core libraries.

Add ENABLE_TESTING option (default ON) to gate Unity/CMock fetch
and test targets for downstream consumers.

tools/CMakeLists.txt

@@ -1,5 +1,4 @@
 add_executable(telemetry telemetry.c)
 target_include_directories(telemetry PRIVATE "${CMAKE_SOURCE_DIR}")
 target_compile_features(telemetry PRIVATE c_std_23)
-target_link_libraries(telemetry PRIVATE celrs_crsf celrs_serial
-                        celrs_serial_platform celrs_logger celrs_log_write)
+target_link_libraries(telemetry PRIVATE cel::cel celserial_platform)