Add yet another JS API library
This PR adds the link to masto.js. There have been already some JS libs, but most of them are not maintained for almost a year, so I've made another one. (originally it were named
lagunehq/core but I've renamed for making it easier to find)
This library supports all APIs until Mastodon 2.8.0-rc1, includes TypeScript definitions, compatible with both browsers and Node.js, so I think it's worth adding to the list.