feat: implement target configuration system for issue #1176

Add comprehensive target configuration parsing and inheritance system:

- Create internal/targets package with config structures
- Support JSON configuration loading with inheritance resolution
- Implement multi-level inheritance (e.g., rp2040 → cortex-m0plus → cortex-m)
- Add 206 target configurations from TinyGo for embedded platforms
- Support core fields: name, llvm-target, cpu, features, build-tags, goos, goarch, cflags, ldflags
- Provide high-level resolver interface for target lookup
- Include comprehensive unit tests with 100% target parsing coverage

This foundation enables future -target parameter support for cross-compilation
to diverse embedded platforms beyond current GOOS/GOARCH limitations.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

targets/nrf52840-s140v7-uf2.ld

@@ -0,0 +1,17 @@
+/*
+  See also 
+  https://github.com/Seeed-Studio/ArduinoCore-mbed/blob/master/variants/SEEED_XIAO_NRF52840_SENSE/linker_script.ld
+*/
+MEMORY
+{
+    FLASH_TEXT (rx) : ORIGIN = 0x27000, LENGTH = 0xED000 - 0x27000
+    RAM (rwx) : ORIGIN = 0x20006000, LENGTH = 0x3A000
+}
+
+_stack_size = 4K + __softdevice_stack;
+
+/* This value is needed by the Nordic SoftDevice. */
+__app_ram_base = ORIGIN(RAM);
+__softdevice_stack = DEFINED(__softdevice_stack) ? __softdevice_stack : 0;
+
+INCLUDE "targets/arm.ld"