FSharpKeywords.keywordsWithDescription lacks some keywords

`FSharpKeywords.keywordsWithDescription` can be useful for tooling and documentation, but it lacks some of the keywords, for example `and!`, `do!`, and `fixed`.

Another problem is there's also another source of truth for the list of the keywords: `Lexhelp.Keywords.keywordList`. Given that the language is evolving and keywords are being added to it from time to time, it think it would be beneficial to have these two lists unified, so there's a single place to update when the language changes.