racket: support racket-langserver lsp (#3808)

* racket: support racket-langserver lsp

* racket-langserver: find highest dir with init.rkt

* autoload/ale/racket: re-indent to 4 spaces

* racket: lint: sort supported tools

* racket: lint: function!

This is _not_ needed anymore, but the lint wants it. See :help E127

* racket-langserver: do not use new dict format

* racket: lint: use snake_case

* add tests for racket-langserver

* racket-langserver tests: correct result values

autoload/ale/racket.vim

@@ -0,0 +1,12 @@
+function! ale#racket#FindProjectRoot(buffer) abort
+    let l:cwd = expand('#' . a:buffer . ':p:h')
+    let l:highest_init = l:cwd
+
+    for l:path in ale#path#Upwards(l:cwd)
+        if filereadable(l:path.'/init.rkt')
+            let l:highest_init = l:path
+        endif
+    endfor
+
+    return l:highest_init
+endfunction