Make phpactor configurable and documented (#5027)

Add php_phpactor_executable for phpactor and make it configurable
through php_phpactor_config.

When phpactor was first added in 3b8ff65, it had no configuration. Thus
the documentation for it could be completely blank. That is why it now
seems like an initial addition

Co-authored-by: cos <cos>

test/linter/test_phpactor.vader

@@ -8,6 +8,12 @@ Execute(The default executable path should be correct):
   AssertLinter 'phpactor',
   \ ale#Escape('phpactor') . ' language-server'
 
+Execute(Overriding the executable should work):
+  let b:ale_php_phpactor_executable = 'my_phpactor'
+
+  AssertLinter 'my_phpactor', ale#Escape('my_phpactor') . ' language-server'
+  unlet b:ale_php_phpactor_executable
+
 Execute(The project path should be correct for .git directories):
   call ale#test#SetFilename('../test-files/php/with-git/test.php')
   silent! call mkdir('../test-files/php/with-git/.git')
@@ -18,3 +24,20 @@ Execute(The project path should be correct for composer.json file):
   call ale#test#SetFilename('../test-files/php/with-composer/test.php')
 
   AssertLSPProject ale#path#Simplify(g:dir . '/../test-files/php/with-composer')
+
+Execute(The LSP values should be set correctly):
+
+  AssertLSPLanguage 'php'
+
+  AssertLSPOptions {}
+
+  AssertLSPConfig {}
+
+Execute(Should accept initialization options):
+  let b:ale_php_phpactor_init_options = {
+  \   'language_server_phpstan.enabled': v:false,
+  \ }
+
+  AssertLSPOptions {
+  \   'language_server_phpstan.enabled': v:false,
+  \ }