Implement support for apkbuild-fixer (#4881)

I am not the author of a single file, but have full permissions from the
original author for permission for submitting this to ALE under the
2-Clause BSD licence.

See: https://gitlab.alpinelinux.org/Leo/apkbuild.vim/-/issues/3

doc/ale-supported-languages-and-tools.txt

@@ -23,6 +23,7 @@ Notes:
 * API Blueprint
   * `drafter`
 * APKBUILD
+  * `apkbuild-fixer`
   * `apkbuild-lint`
   * `secfixes-check`
 * AsciiDoc