Fixes in generated TypeScript code

[mjambon]

• atdts: Fix incorrect type for TypeScript readers/writers generated for type abstract.
• atdts: Fix incorrect type for TypeScript writers of optional fields. It was working only in special cases such as foo?: int.
• atdts: Eliminate the type alias type Int = number since it was more confusing than helpful. Occurrences of Int are replaced by number /*int*/.

This eliminates errors produced by tsc when compiling the TypeScript code derived from semgrep_output_v1.atd.

PR checklist

• [x] New code has tests to catch future regressions
• [x] Documentation is up-to-date
• [x] CHANGES.md is up-to-date