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/esp8266.json

@@ -0,0 +1,18 @@
+{
+	"inherits": ["xtensa"],
+	"cpu": "esp8266",
+	"features": "+debug,+density,+exception,+extendedl32r,+highpriinterrupts,+interrupt,+mul32,+nsa,+prid,+regprotect,+rvector,+timerint",
+	"build-tags": ["esp8266", "esp"],
+	"scheduler": "tasks",
+	"linker": "ld.lld",
+	"default-stack-size": 2048,
+	"rtlib": "compiler-rt",
+	"libc": "picolibc",
+	"linkerscript": "targets/esp8266.ld",
+	"extra-files": [
+		"src/device/esp/esp8266.S",
+		"src/internal/task/task_stack_esp8266.S"
+	],
+	"binary-format": "esp8266",
+	"flash-command": "esptool.py --chip=esp8266 --port {port} write_flash 0x00000 {bin} -fm qio"
+}