Fix JS bind expando declaration emit

[cjc0013]

Fixes #4045

This updates declaration emit for JS variables initialized from fn.bind({}) so the exported alias is emitted as a function declaration based on the bound target signature instead of const alias: typeof fn. That avoids leaking non-exported implementation functions into the generated .d.ts.

It also carries expando properties from the bound source function onto the public alias, while letting alias-owned expando assignments provide their own property declarations.

Tests:

• go build -tags=noembed -o .\built\local\tsgo.exe .\cmd\tsgo
• go test ./internal/testrunner/ -run 'TestLocal/(declarationEmitExpandoFunction|declarationEmitExpandoArrowFunctionParameter|functionExpandoPropertyDeclaration|jsExpandoAssignmentElementAccess|declarationEmitJsBindExpandoFunction)' -v
• go test ./internal/transformers/declarations
• git diff --check

AI disclosure: I used Codex to help investigate and draft this change; I reviewed the diff and test output and will respond to feedback myself.