New linter: dls (#1992)

* New linter: dls

ale_linters/d/dls.vim

@@ -0,0 +1,22 @@
+" Author: aurieh <me@aurieh.me>
+" Description: A Language Server implementation for D
+
+call ale#Set('d_dls_executable', 'dls')
+
+function! ale_linters#d#dls#GetExecutable(buffer) abort
+    return ale#Var(a:buffer, 'd_dls_executable')
+endfunction
+
+function! ale_linters#d#dls#FindProjectRoot(buffer) abort
+    " Note: this will return . if dub config is empty
+    " dls can run outside DUB projects just fine
+    return fnamemodify(ale#d#FindDUBConfig(a:buffer), ':h')
+endfunction
+
+call ale#linter#Define('d', {
+\   'name': 'dls',
+\   'lsp': 'stdio',
+\   'executable_callback': 'ale_linters#d#dls#GetExecutable',
+\   'command_callback': 'ale_linters#d#dls#GetExecutable',
+\   'project_root_callback': 'ale_linters#d#dls#FindProjectRoot',
+\})