add --keepDecorators compile option

[kingsimba]

As #22980 has pointed out. There is a sound logic for lossing decorators in declaration file ".d.ts".

But in my opinion, some decorators do provide useful information for readers. For example:

@expose({name: "sockFile", since: 2.0})
domainSocketFile: string

It tells me that when serialize to JSON, it will be renamed to "sockFile", and it will only be serialized after version 2.0

Suggestion

I'd suggest
keepDecorators: never = default
keepDecorators: declaration = put decorator in .d.ts files. Perhaps in comments?

[kingsimba]

I think "design time decorator" #2900 can fit my need.