BridgeJS: Lower imported optional stack parameters fully on the stack

[krodak]

Summary

Imported optional parameters with stack-only payloads ([T]?, [String: V]?, @JS struct?) used a hybrid convention: the isSome flag crossed as a wasm i32 argument while the payload went on the shared stacks. Optional returns and optional array elements of the same types already put both on the stacks.

This lowers those parameters the same way. The wasm signature carries no argument for them; JS pops the flag and conditionally lifts, through the same fragment as optional returns. Scalars, strings, JSObject, closures, enums and heap objects keep their current optional ABIs, as do all exported signatures.

// before: flag as a wasm argument, payload on the stacks
let aIsSome = a.bridgeJSLowerParameter()
let ret = bjs_f(aIsSome)      // JS: function bjs_f(a) { if (a) { ...lift... } }

// after: all-stack
let _ = a.bridgeJSLowerParameter()
let ret = bjs_f()             // JS: pops flag, conditionally lifts

The hybrid shape was the last parameter kind that both passed a wasm argument and pushed stack data, which is what enabled the argument transposition fixed in swiftwasm#794. After this, every stack-touching parameter is flagless and reverse-ordered.

Test plan

• BridgeJS plugin tests / snapshots (8 snapshot files, all imported optional-stack shapes)
• make unittest: ImportAPITests.testStackLoweredParameterOrder, optional round-trips with some and nil
• check:bridgejs-dts, formatter, no generated drift