feat(stdlib): Add Mimetypes module

[JeysonFlores]

Hey there! This is my first contribution to the Grain project so I wanted to start with something easy to write, debug and review. This module is just a collection of mimetypes and formats with some entrypoints to access to them with a pretty general default behaviour.

Details:

• The list of MIMETypes and format is extracted from this project which itself is composed using the media types from Apache, IANA and NGINX.
• The getters return "text/plain" and "txt" when a format or mimetype is not found. This is a pretty standard behaviour for multiple standard libraries across many programming languages.

I hope the value of this module increases in the future when networking modules are added to the standard library. By the time being I think this is a good start.

Thanks!

Note: I just put an arbitrary version in the source code. Feel free to change it

[ospencer]

Very cool! We'll discuss this tomorrow during our community call—it'd be great if you could make it (we have them on Discord—you should be able to see the event there)!

[ospencer]

To recap what we discussed on the community call: we don't think this library makes sense to be a part of the standard library, but it would make an awesome community package! If you made this its own repository, it'd be great to feature it in https://github.com/grain-lang/awesome-grain!