feat: support Bazel buildifier linter (#4529)

* Initial buildifier linter files

* Add handler test

* Fix test when options are not set

test/handler/test_buildifier_handler.vader

@@ -0,0 +1,26 @@
+Before:
+  runtime ale_linters/bzl/buildifier.vim
+
+After:
+  call ale#linter#Reset()
+
+Execute(The buildifier handler should parse lines correctly):
+  AssertEqual
+  \ [
+  \   {
+  \     'lnum': 26,
+  \     'col': 1,
+  \     'type': 'E',
+  \     'text': 'syntax error near'';'' and move the next statement to the new line',
+  \   },
+  \   {
+  \     'lnum': 7,
+  \     'col': 0,
+  \     'type': 'W',
+  \     'text': 'unused-variable: Variable "foo" is unused. Please remove it. (https://github.com/bazelbuild/buildtools/blob/master/WARNINGS.md#unused-variable)'
+  \   },
+  \ ],
+  \ ale_linters#bzl#buildifier#Handle(bufnr(''), [
+  \   'swiftformat/toolchains/assets.bzl:26:1: syntax error near'';'' and move the next statement to the new line',
+  \   'swiftformat/toolchains/assets.bzl:7: unused-variable: Variable "foo" is unused. Please remove it. (https://github.com/bazelbuild/buildtools/blob/master/WARNINGS.md#unused-variable)',
+  \ ])

test/linter/test_bzl_buildifier.vader

@@ -0,0 +1,30 @@
+Before:
+  call ale#assert#SetUpLinterTest('bzl', 'buildifier')
+
+After:
+  call ale#assert#TearDownLinterTest()
+
+
+Execute(Should use default command when bazel_buildifier_options are not set):
+  call ale#test#SetDirectory('/testplugin/test/test-files/bzl/bazel-package')
+  call ale#test#SetFilename('BUILD.bazel')
+
+  let g:ale_bazel_buildifier_executable = 'buildifier'
+  let g:ale_bazel_buildifier_options = ''
+
+  AssertLinter 'buildifier',
+  \ ale#Escape('buildifier') . ' -mode check -lint warn -path %s'
+
+  call ale#test#RestoreDirectory()
+
+Execute(Should use custom buildifier options when bazel_buildifier_options are set):
+  call ale#test#SetDirectory('/testplugin/test/test-files/bzl/bazel-package')
+  call ale#test#SetFilename('BUILD.bazel')
+
+  let g:ale_bazel_buildifier_executable = 'buildifier'
+  let g:ale_bazel_buildifier_options = '-v'
+
+  AssertLinter 'buildifier',
+  \ ale#Escape('buildifier') . ' -mode check -lint warn -path %s -v'
+
+  call ale#test#RestoreDirectory()