Add support for c3-lsp linter (#4836)

* Add support for c3-lsp linter

Add support for c3-lang with the c3-lsp language server.

Link: http://github.com/pherrymason/c3-lsp
Link: http://c3-lang.org

* fix linter error

* fix: consistent use of the executable name

Consistently use the executable name 'c3lsp' instead of the project name
'c3-lsp'.

* c3lsp: add command line arguments to executable

autoload/ale/handlers/c3lsp.vim

@@ -0,0 +1,19 @@
+scriptencoding utf-8
+" Author: Koni Marti <koni.marti@gmail.com>
+" Description: Utilities for c3lsp
+
+function! ale#handlers#c3lsp#GetProjectRoot(buffer) abort
+    let l:config = ale#path#FindNearestFile(a:buffer, 'project.json')
+
+    if !empty(l:config)
+        return fnamemodify(l:config, ':h')
+    endif
+
+    return expand('#' . a:buffer . ':p:h')
+endfunction
+
+function! ale#handlers#c3lsp#GetInitOpts(buffer, init_options_var) abort
+    let l:init_options = {}
+
+    return extend(l:init_options, ale#Var(a:buffer, a:init_options_var))
+endfunction