Add better support for NimScript files

[PMunch]

Currently NimScript files are parsed as-is by NimLSP. This is a problem with e.g. nimble files that are Nim, but has some extra imports and such added before they are executed. My idea to solve this is to introduce a folder of scripts on the form nimble.nimlsp where the filename would match the file extension to apply to and the nimlsp extension to differentiate them from other Nim/-Script files. When a file with an extension found in this folder is opened NimLSP will run a callback from that file to modify the content before each operation, and it would run another callback to modify the row/column information before handing the information back to the user. This way users of Nim could easily create these editor files for whatever NimScript format they create, without the need to hard-code anything into NimLSP.