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

test/fixers/test_opa_fmt_fixer_callback.vader

@@ -0,0 +1,33 @@
+Before:
+  Save g:ale_opa_fmt_executable
+  Save g:ale_opa_fmt_options
+
+  " Use an invalid global executable, so we don't match it.
+  let g:ale_opa_fmt_executable = 'xxxinvalid'
+  let g:ale_opa_fmt_options = ''
+
+  call ale#test#SetDirectory('/testplugin/test/fixers')
+
+After:
+  Restore
+
+  call ale#test#RestoreDirectory()
+
+Execute(The opa fmt callback should return the correct default values):
+
+  AssertEqual
+  \ {
+  \   'command': ale#Escape('xxxinvalid') . ' fmt',
+  \ },
+  \ ale#fixers#opafmt#Fix(bufnr(''))
+
+Execute(The opa fmt callback should include custom options):
+  let g:ale_opa_fmt_options = "--list"
+
+  AssertEqual
+  \ {
+  \   'command': ale#Escape('xxxinvalid')
+  \     . ' fmt'
+  \     . ' ' . g:ale_opa_fmt_options
+  \ },
+  \ ale#fixers#opafmt#Fix(bufnr(''))

test/linter/test_rego_opacheck.vader

@@ -0,0 +1,16 @@
+
+" Based upon :help ale-development
+Before:
+  call ale#assert#SetUpLinterTest('rego', 'opacheck')
+
+After:
+  call ale#assert#TearDownLinterTest()
+
+Execute(The default command should be correct):
+  AssertLinter 'opa',
+  \ ale#Escape('opa') . ' check %s --format json '
+
+Execute(The default command should be overriden):
+  let b:ale_rego_opacheck_executable = '/bin/other/opa'
+  AssertLinter '/bin/other/opa',
+  \ ale#Escape('/bin/other/opa') . ' check %s --format json '