Add buf linter and fixer (#4128)

* Add buf lint to linters

* Add buf format to fixers

* Fix test/linter/test_buf_lint.vader

* Fix test/fixers/test_buf_format_fixer_callback.vader

* Simplify test/test-files/proto/testfile.proto

* Add buf-lint alias and rename linter

autoload/ale/fix/registry.vim

@@ -37,6 +37,11 @@ let s:default_registry = {
 \       'suggested_filetypes': ['python'],
 \       'description': 'Fix PEP8 issues with black.',
 \   },
+\   'buf-format': {
+\       'function': 'ale#fixers#buf_format#Fix',
+\       'suggested_filetypes': ['proto'],
+\       'description': 'Fix .proto files with buf format.',
+\   },
 \   'buildifier': {
 \       'function': 'ale#fixers#buildifier#Fix',
 \       'suggested_filetypes': ['bzl'],