Add template directives support (%{if}, %{for}) in quoted strings

[kkozik-amplify]

Summary

• Adds parsing support for HCL2 template directives (%{if}, %{else}, %{endif}, %{for}, %{endfor}) within quoted strings, including strip markers (~) and nested directives
• Grammar uses flat directive rules assembled into nested structures by the transformer to avoid LALR conflicts
• Full round-trip support: parse, serialize, deserialize, reconstruct all handle directives correctly

Caused by #247

Test plan

• 27 new unit tests for all directive rule classes
• 5 new integration tests (HCL→JSON, reconstruct, reserialization, JSON→HCL, full round-trip)
• All 735 tests pass
• Parses the exact input from Parsing error when comparing strings within a string template directive #247: "kms%{ if var.id != \"primary\" }-${var.id}%{ endif }"

🤖 Generated with Claude Code