Client gen code support: JavaScript

[bih]

We currently support TypeScript, but for older clients, they may want an untyped version if they don't use TypeScript at all in their codebase.

See code files here:

• https://github.com/encoredev/encore/blob/main/cli/internal/codegen/typescript.go
• https://github.com/encoredev/encore/blob/main/cli/internal/codegen/testdata/expected_typescript.ts
• https://github.com/encoredev/encore/blob/main/cli/internal/codegen/testdata/expected_noauth_typescript.ts