Add support for phpdoc tag @used-by

[alexr00]

I see there is support for tag @uses but there is no support for @used-by even though both are valid phpdoc tags.

As a result, tag @used-by does not get highlighting and is not included in the preview of the function:

/**
 * @uses empty() Just a test
 * @used-by anotherFunction() Just a description
 * @return void 
 */
function sample() {
  empty(123);
}

Source code links:

• https://github.com/microsoft/vscode/blob/ec4019f7b61fd66b6aef685ba43087a73f5bc9b8/extensions/php/syntaxes/php.tmLanguage.json#L2791
• https://github.com/microsoft/vscode/blob/ec4019f7b61fd66b6aef685ba43087a73f5bc9b8/extensions/php/syntaxes/php.tmLanguage.json#L3568

Originally from @alexkuc in microsoft/vscode#227717