Map file extension to content type

[Cre3per]

https://github.com/denoland/deno/pull/17172 adds --ext to deno, eg.

deno run --ext js mod

--ext needs to map file extensions to mime types

The mapping function in deno is incomplete and temporary.

deno_ast's MediaType already maps module specifiers and mime types to MediaType. I think this is a good place to map

• file extensions to MediaType
• MediaType to mime types

[Cre3per]

I'd like to work on this.

There is no 1:1 mapping between file extensions and mime types, eg. application/typescript and text/typescript both have a ".ts" file extension. For a start, I'll write a mapping function based on the mime types in from_content_type()

[dsherret]

Sounds good to me!