feat: add scopes command and CLI integration tests

Add `srcmap scopes` command to inspect ECMA-426 scopes and variable
bindings in source maps, with both human-readable and --json output.

Add 32 integration tests covering all CLI commands (info, validate,
lookup, resolve, decode, encode, mappings, concat, remap, symbolicate,
scopes, schema) including error handling and stdin support.

Fix README commands table to include symbolicate and scopes.

Author: BartWaardenburg