add rego support (#4199)

* add opa fmt fixer for rego files

* add opa linter

* add basic tests for linter and fixer

* add cspell to the docs

autoload/ale/fix/registry.vim

@@ -526,6 +526,11 @@ let s:default_registry = {
 \       'suggested_filetypes': ['pascal'],
 \       'description': 'Fix Pascal files with ptop.',
 \   },
+\   'opafmt': {
+\       'function': 'ale#fixers#opafmt#Fix',
+\       'suggested_filetypes': ['rego'],
+\       'description': 'Fix rego files with opa fmt.',
+\   },
 \   'vfmt': {
 \       'function': 'ale#fixers#vfmt#Fix',
 \       'suggested_filetypes': ['v'],