False positive (javascript): "Invalid regular expression flag" for v flag

**Description of the false positive**

`v` is a valid regular expression flag

**Code samples or links to source code**

```js
let x = /abc/v;
```

https://github.com/mozilla/pdf.js/blob/d88035525c69b72ecec2f5ea21b591d2d541a2b5/web/autolinker.js#L112

**URL to the alert on GitHub code scanning (optional)**

Start with the JavaScript sample and the referenced web/autolinker.js line 112 in pdf.js, then reproduce the code-scanning alert for the /abc/v expression. Trace which regular-expression flag handling produces the warning and verify that the valid v flag no longer triggers it.

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