Generate TypeScript "bigint" type instead of "BigInt"

[mjoblin]

Description of Changes

Generate the TypeScript bigint type instead of BigInt (context from the TS docs).

API and ABI breaking changes

This is probably not a notable API change, although it's technically tweaking a type definition in code generated for client use so I'm calling that out here just in case.

Expected complexity level and risk

2

I'm only marking this as a 2 and not a 1 because it relates to another PR in clockworklabs/spacetimedb-typescript-sdk. I don't believe there's a release dependency between these two PRs.

Testing

cargo test still passes. I also tried this change locally and it worked as expected for my u64 case, where it generated bigint types where expected and the code still ran fine in the browser with no TypeScript warnings/errors.

I haven't added any tests to test specifically for bigint generation.

• [x] cargo test (including test_typescript_codegen_output)