Adds PHPActor Linter (LSP) (#3975)

* Adds phpactor lsp linter

* Fixes missing comma

* Adds tests for phpactor lsp linter

* Adds note that this part is not my own work

* Removes unused variable

* Adds phpactor to supported tools list

* Fixes doc sorting

* Wraps phpactor in code tags

test/linter/test_phpactor.vader

@@ -0,0 +1,20 @@
+Before:
+  call ale#assert#SetUpLinterTest('php', 'phpactor')
+
+After:
+  call ale#assert#TearDownLinterTest()
+
+Execute(The default executable path should be correct):
+  AssertLinter 'phpactor',
+  \ ale#Escape('phpactor') . ' language-server'
+
+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')
+
+  AssertLSPProject ale#path#Simplify(g:dir . '/../test-files/php/with-git')
+
+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')