Add linter for Inko

This adds a linter for Inko (https://inko-lang.org/). The linter makes
use of Inko's own compiler, and a newly introduced --check flag to only
check for errors; instead of also compiling source code.

autoload/ale/handlers/inko.vim

@@ -0,0 +1,37 @@
+" Author: Yorick Peterse <yorick@yorickpeterse.com>
+" Description: output handlers for the Inko JSON format
+
+function! ale#handlers#inko#GetType(severity) abort
+    if a:severity is? 'warning'
+        return 'W'
+    endif
+
+    return 'E'
+endfunction
+
+function! ale#handlers#inko#Handle(buffer, lines) abort
+    try
+        let l:errors = json_decode(join(a:lines, ''))
+    catch
+        return []
+    endtry
+
+    if empty(l:errors)
+        return []
+    endif
+
+    let l:output = []
+    let l:dir = expand('#' . a:buffer . ':p:h')
+
+    for l:error in l:errors
+        call add(l:output, {
+        \   'filename': ale#path#GetAbsPath(l:dir, l:error['file']),
+        \   'lnum': l:error['line'],
+        \   'col': l:error['column'],
+        \   'text': l:error['message'],
+        \   'type': ale#handlers#inko#GetType(l:error['level']),
+        \})
+    endfor
+
+    return l:output
+endfunction

autoload/ale/linter.vim

@@ -43,6 +43,7 @@ let s:default_ale_linters = {
 \   'go': ['gofmt', 'golint', 'go vet'],
 \   'hack': ['hack'],
 \   'help': [],
+\   'inko': ['inko'],
 \   'perl': ['perlcritic'],
 \   'perl6': [],
 \   'python': ['flake8', 'mypy', 'pylint', 'pyright'],