Make persistent/temporary/instance keywords target-aware (fixes #1847)

[Sukuna0007Abhi]

Fixes #1847

PR #1664 added persistent, temporary, and instance as global keywords, which breaks
any Solidity code using these as identifiers on non-Soroban targets (e.g. uint256 persistent = 0;).

This fix makes the lexer target-aware, as suggested by @seanyoung. The three words are only
treated as keywords when targeting Soroban, and fall back to plain identifiers for all other targets.

Changes:

• solang-parser/src/lexer.rs — added soroban: bool flag; keyword lookup skips the three tokens when flag is off
• solang-parser/src/lib.rs — added parse_soroban() alongside existing parse(); shared logic in parse_internal()
• src/sema/mod.rs — dispatches to parse_soroban() when ns.target == Soroban, parse() otherwise
• solang-parser/src/tests.rs — two regression tests: identifiers pass in default, keywords pass in soroban

Testing:

• All 18 parser unit tests pass
• All 4 doc-tests pass
• cargo fmt --check clean