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-s140v6-uf2.ld

@@ -0,0 +1,13 @@
+
+MEMORY
+{
+    FLASH_TEXT (rw) : ORIGIN = 0x00000000+0x26000, LENGTH = 0xED000-0x26000 /* SoftDevice S140. See https://learn.adafruit.com/introducing-the-adafruit-nrf52840-feather/hathach-memory-map. Application starts at 0x26000; user data starts at 0xED000 */
+    RAM (xrw)       : ORIGIN = 0x20004180, LENGTH = 0x20040000-0x20004180
+}
+
+_stack_size = 2K;
+
+/* This value is needed by the Nordic SoftDevice. */
+__app_ram_base = ORIGIN(RAM);
+
+INCLUDE "targets/arm.ld"