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/rp2040.json

@@ -0,0 +1,23 @@
+{
+    "inherits": ["cortex-m0plus"],
+    "build-tags": ["rp2040", "rp"],
+    "scheduler": "cores",
+    "flash-1200-bps-reset": "true",
+    "flash-method": "msd",
+    "serial": "usb",
+    "msd-volume-name": ["RPI-RP2"],
+    "msd-firmware-name": "firmware.uf2",
+    "binary-format": "uf2",
+    "uf2-family-id": "0xe48bff56",
+    "rp2040-boot-patch": true,
+    "extra-files": [
+        "src/device/rp/rp2040.s"
+    ],
+    "ldflags": [
+        "--defsym=__num_stacks=2"
+    ],
+    "linkerscript": "targets/rp2040.ld",
+    "openocd-interface": "picoprobe",
+    "openocd-transport": "swd",
+    "openocd-target": "rp2040"
+}