Add a JS config file mode.

[bbkane]

The "directory" linking mode currently implemented doesn't support apps wanting their config in different places depending on the OS, or the user wanting to link a specific config depending on the hostname, etc.

It might be pure over engineering fun to embed a scripting language that the user can use to produce a list of links.

API:

fling config
    create # create config with type skeletons 
    eval # eval the config and print list of links 
    link
    unlink 

Things I want from the scripting language:

• a pure go library
• easy FFI
• sandboxed
• IDE support

I think GoJa with JSDoc comments + VS Code satisfy these, or perhaps Lua with LuaLSP type hints, or perhaps Typescript with whatever Logdy does

I also need to embed the version in the config so I can print a migration guide link if the API fling expects changes