feat(cli): accept standard json input on stdin

[malsomesh9]

Summary

Teach solang compile --standard-json to accept Solidity standard JSON input from stdin, compile the declared sources, and print the existing standard JSON output shape.

Changes

• parse standard JSON input from stdin when --standard-json is used without positional source files
• load inline source content and local urls entries into the file resolver
• apply settings.remappings to import resolution
• restore standard JSON contract grouping by source file and add a CLI regression test

Fixes #791

Verification

• cargo fmt --all
• attempted cargo test --test cli standard_json_reads_sources_from_stdin but local build is blocked here by a missing system LLVM toolchain required by llvm-sys