fix: wire up with_meta to emit __start_line__/__end_line__

[bejaratommy]

Summary

The with_meta serialization option — and the hcl2tojson --with-meta CLI flag — has been a no-op since the v8 architecture overhaul. The flag is accepted and documented ("Add __start_line__ and __end_line__ metadata to each block/attribute"), but no serialize() path ever reads options.with_meta, so the output never contains line metadata.

$ echo 'x = 1' | hcl2tojson - --with-meta
{"x": 1}                      # before
{"x": 1, "__start_line__": 1, "__end_line__": 1}   # after

Change

Lark already runs with propagate_positions=True and the transformer attaches a populated Meta to every rule, so the line numbers are available — they were just never written out.

BodyRule.serialize now emits __start_line__ / __end_line__ on each body dict (the document body and each block body) when with_meta is set, sourced from the rule's propagated Meta. This is a single, uniform injection point that covers both the top-level-attribute case above and block bodies. The keys are added as constants in hcl2/const.py.

The metadata is only emitted when with_meta=True (default False), so default output is unchanged. The empty-Meta case (trees built without positions, e.g. via the deserializer) is guarded, so nothing is emitted there.

Tests

Added test/unit/test_api.py cases asserting the metadata appears with with_meta=True (top-level attribute and block body) and is absent by default.

Fixes amplify-education#291