Add _ide_helpers.php to git export-ignore

This file actually interferes with application-level LSP, because LSP
picks it up and indexes it, then things break like goto definition and
results in warnings about missing methods like `Route::get()`.

Fixes  #557

.gitattributes

@@ -15,3 +15,5 @@
 /CHANGELOG.md       export-ignore
 /CONTRIBUTING.md    export-ignore
 /README.md          export-ignore
+/.php-cs-fixer.dist export-ignore
+/_ide_helpers.php   export-ignore