Uart shared variant emitter

[crabel99]

Summary

Refactors UART variant setup so Uart.cpp owns the common Serial* object and interrupt-handler emission, matching the structure already used by Wire and SPI.

Changes

• Adds shared UART emitters for Serial, Serial1 through Serial5, and SerialUART.
• Derives UART SERCOM interrupt handlers from each variant's PERIPH_SERIAL* macro.
• Removes duplicated Uart Serial* object definitions from variant .cpp files.
• Removes duplicated SERIAL*_IT_HANDLER definitions from variant headers where the core can derive them.
• Keeps variant-owned UART opt-outs for special cases such as gemma_m0.

Validation

• git diff --check -- cores/arduino/Uart.cpp variants
• python3 tools/build_all.py metro_m0
• python3 tools/build_all.py metro_m4
• python3 tools/build_all.py matrixportal_m4