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/stm32f4disco.json

@@ -0,0 +1,12 @@
+{
+  "inherits": ["cortex-m4"],
+  "build-tags": ["stm32f4disco", "stm32f407", "stm32f4", "stm32"],
+  "serial": "uart",
+  "linkerscript": "targets/stm32f407.ld",
+  "extra-files": [
+    "src/device/stm32/stm32f407.s"
+  ],
+  "flash-method": "openocd",
+  "openocd-interface": "stlink-v2",
+  "openocd-target": "stm32f4x"
+}