feat: add inheritable-only field to filter processor-level targets (#5270)

* feat: add inheritable-only field to hide processor-level targets from listing

Processor-level targets like esp32, rp2040, etc. have flash-method set
so they leak through the existing heuristic filter in GetTargetSpecs and
appear in `tinygo targets` output. Add an "inheritable-only" JSON field
that is checked on raw JSON before inheritance resolution, preventing
these non-board targets from being listed while keeping them loadable
for direct builds and inheritance.

Ref: tinygo-org/tinygo#5178

* fix: prevent inheritable-only from propagating to child targets

The InheritableOnly bool field was propagating from parent to child
targets through overrideProperties, which would hide board targets
from GetTargetSpecs. Fix by preserving/restoring the field across
resolveInherits. Also simplify GetTargetSpecs by removing the raw
JSON pre-check workaround and remove nonexistent esp32c6 from tests.

* prevent build commands to use inheritable only targets

targets/esp32.json

@@ -1,5 +1,6 @@
 {
 	"inherits": ["xtensa"],
+	"inheritable-only": true,
 	"cpu": "esp32",
 	"features": "+atomctl,+bool,+clamps,+coprocessor,+debug,+density,+dfpaccel,+div32,+exception,+fp,+highpriinterrupts,+interrupt,+loop,+mac16,+memctl,+minmax,+miscsr,+mul32,+mul32high,+nsa,+prid,+regprotect,+rvector,+s32c1i,+sext,+threadptr,+timerint,+windowed",
 	"build-tags": ["esp32", "esp"],