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/atsamd51j20a.json

@@ -0,0 +1,10 @@
+{
+	"inherits": ["cortex-m4"],
+	"build-tags": ["sam", "atsamd51", "atsamd51j20", "atsamd51j20a"],
+	"linkerscript": "targets/atsamd51j20a.ld",
+	"extra-files": [
+		"src/device/sam/atsamd51j20a.s"
+	],
+	"openocd-transport": "swd",
+	"openocd-target": "atsame5x"
+}