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>

ale_linters/php/phpactor.vim

@@ -1,6 +1,9 @@
 " Author: Arizard <https://github.com/Arizard>
 " Description: PHPactor integration for ALE
 
+call ale#Set('php_phpactor_executable', 'phpactor')
+call ale#Set('php_phpactor_init_options', {})
+
 " Copied from langserver.vim
 function! ale_linters#php#phpactor#GetProjectRoot(buffer) abort
     let l:composer_path = ale#path#FindNearestFile(a:buffer, 'composer.json')
@@ -17,7 +20,8 @@ endfunction
 call ale#linter#Define('php', {
 \   'name': 'phpactor',
 \   'lsp': 'stdio',
-\   'executable': 'phpactor',
+\   'executable': {b -> ale#Var(b, 'php_phpactor_executable')},
 \   'command': '%e language-server',
+\   'initialization_options': {b -> ale#Var(b, 'php_phpactor_init_options')},
 \   'project_root': function('ale_linters#php#phpactor#GetProjectRoot'),
 \})