[v3] Tags and key features

[angelikatyborska]

closes #27, closes #24, closes #25

For the key features, I took the 4 key features from https://gleam.run/ and spread them out into 6. The descriptions had to be no longer than 100 chars. Icons are not available yet, I'll submit another PR once icons were finalized.

For the tags, I took Elixir's tags, changed dynamic typing to static typing, and removed all "used for" tags. AFAIK Gleam isn't used in production in any significant way ✨ yet ✨, so I thought those tags are not appropriate at this stage. For reference, all available tags: https://github.com/exercism/docs/blob/main/building/tracks/config-json.md#tags

For the configlet linter check, I had to rename some files. I hope I did that correctly. I have no prior Gleam experience (sorry 😢). I also didn't find any instructions in the repository on how to verify if exercises still work. I guess this is because the track is not active yet.

[lpil]

Wonderful! Thank you so much!