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

test/fixers/test_buf_format_fixer_callback.vader

@@ -0,0 +1,21 @@
+Before:
+  Save g:ale_proto_buf_format_executable
+
+  " Use an invalid global executable, so we don't match it.
+  let g:ale_proto_buf_format_executable = 'xxxinvalid'
+
+  call ale#test#SetDirectory('/testplugin/test/fixers')
+
+After:
+  Restore
+
+  call ale#test#RestoreDirectory()
+
+Execute(The buf-format callback should return the correct default values):
+  call ale#test#SetFilename('../test-files/proto/testfile.proto')
+
+  AssertEqual
+  \ {
+  \   'command': ale#Escape('xxxinvalid') . ' format %t',
+  \ },
+  \ ale#fixers#buf_format#Fix(bufnr(''))

test/linter/test_buf_lint.vader

@@ -0,0 +1,22 @@
+Before:
+  call ale#assert#SetUpLinterTest('proto', 'buf_lint')
+  call ale#test#SetFilename('test.proto')
+
+After:
+  call ale#assert#TearDownLinterTest()
+
+Execute(The default command should be correct):
+  AssertLinter 'buf',
+  \ ale#Escape('buf')
+  \ . ' lint'
+  \ . ' %s#include_package_files=true'
+
+Execute(The callback should include any additional options):
+  let b:ale_proto_buf_lint_executable = '/tmp/buf'
+  let b:ale_proto_buf_lint_config = '/tmp/buf.yaml'
+
+  AssertLinter '/tmp/buf',
+  \ ale#Escape('/tmp/buf')
+  \ . ' lint'
+  \ . ' --config=' . ale#Escape('/tmp/buf.yaml')
+  \ . ' %s#include_package_files=true'