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/attiny1616.json

@@ -0,0 +1,14 @@
+{
+    "inherits": ["avrtiny"],
+    "cpu": "attiny1616",
+    "build-tags": ["attiny1616"],
+    "gc": "none",
+    "cflags": [
+        "-D__AVR_ARCH__=103"
+    ],
+    "linkerscript": "src/device/avr/attiny1616.ld",
+    "extra-files": [
+        "src/device/avr/attiny1616.s"
+    ],
+    "flash-command": "pymcuprog write -f {hex} --erase --verify -d attiny1616 -t uart -u {port}"
+}