Add deadnix linter (#4443)

test/handler/test_deadnix_handler.vader

@@ -0,0 +1,27 @@
+Execute(The deadnix handler should handle deadnix output):
+  let output = [
+  \'{"file":"./flake.nix","results":[{"column":5,"endColumn":9,"line":23,"message":"Unused lambda pattern: self"},{"column":2,"endColumn":6,"line":1,"message":"Unused lambda pattern: pkgs"}]}'
+  \]
+
+  AssertEqual
+  \ [
+  \   {
+  \     'lnum': 23,
+  \     'col': 5,
+  \     'end_col': 9,
+  \     'text': 'Unused lambda pattern: self',
+  \     'type': 'W',
+  \   },
+  \   {
+  \     'lnum': 1,
+  \     'col': 2,
+  \     'end_col': 6,
+  \     'text': 'Unused lambda pattern: pkgs',
+  \     'type': 'W',
+  \   },
+  \ ],
+  \ ale#handlers#deadnix#Handle(bufnr(''), output)
+
+  AssertEqual [], ale#handlers#deadnix#Handle(bufnr(''), [''])
+  AssertEqual [], ale#handlers#deadnix#Handle(bufnr(''), ['not json'])
+  AssertEqual [], ale#handlers#deadnix#Handle(bufnr(''), ['{"results":[{}]}'])

test/linter/test_nix_deadnix.vader

@@ -0,0 +1,19 @@
+Before:
+  call ale#assert#SetUpLinterTest('nix', 'deadnix')
+
+After:
+  call ale#assert#TearDownLinterTest()
+
+Execute(The deadnix command should be correct):
+  AssertLinter 'deadnix', ale#Escape('deadnix') . ' -o json -- %t'
+
+Execute(Additional deadnix options should be configurable):
+  let g:ale_nix_deadnix_options = '--foobar'
+
+  AssertLinter 'deadnix',
+  \ ale#Escape('deadnix') . ' -o json --foobar -- %t'
+
+Execute(The deadnix command should be configurable):
+  let g:ale_nix_deadnix_executable = 'foo/bar'
+
+  AssertLinter 'foo/bar', ale#Escape('foo/bar') . ' -o json -- %t'