feat: support PlatformIO monitor_filters

[zackees]

Context

PlatformIO supports monitor_filters in platformio.ini, but fbuild does not currently expose it as a first-class setting in the board/config model or generated PlatformIO output.

This matters for ESP32-family boards because the serial monitor output often needs PlatformIO's esp32_exception_decoder filter to turn panic/backtrace output into useful diagnostics.

Proposal

Add monitor_filters support to fbuild's PlatformIO compatibility layer.

Suggested behavior:

• Add a nullable/list field for monitor filters in the relevant config model.
• Preserve explicit user-provided filters when parsing/generating platformio.ini.
• For ESP32-family boards, default to default, esp32_exception_decoder when the field is unset.
• Allow an explicit empty list to suppress the ESP32 auto-default.
• Document the supported key in docs/reference/platformio-ini.md.

Acceptance criteria

• fbuild can emit monitor_filters in generated platformio.ini output.
• ESP32-family boards default to monitor_filters = default, esp32_exception_decoder when unset.
• Explicit monitor_filters = [] / equivalent config suppresses the auto-default and emits no monitor filter entry, or the chosen empty behavior is documented clearly.
• Custom filter lists are emitted exactly as configured.
• Non-ESP32 boards do not receive ESP32 monitor filters by default.
• Tests cover ESP32 default, explicit empty suppression, custom filters, and non-ESP32 default behavior.