[html] don't validate custom event handlers as JavaScript

- VSCode Version: 1.37.1 - OS Version: Windows 10 **Consider the following code snippet:** `<div onInvalidAttribute="test test" otherInvalidAttributeWithoutOnPrefix="test test"></div>` If the name of the attribute is prefixed with **“on”** and its value contains an empty space, then the following error is shown on the UI: “‘;’ expected. (js)” and in the console output: ``` { "resource": "/d:/test ext/test.html", "owner": "_generated_diagnostic_collection_name_#0", "severity": 8, "message": "';' expected.", "source": "js", "startLineNumber": 1, "startColumn": 31, "endLineNumber": 1, "endColumn": 35 } ``` I believe vs code thinks the attribute is an event or special in some way because it begins with “on” and performs some stricter validations on its value. **Steps to Reproduce:** 1. Create HTML file 2. Add any kind of html tag 3. Add an attribute with “on” prefix to that tag (**onInvalidAttribute** was used in this example) 4. Give it a value containing an empty space in between (**test test** was used in this example) Does this issue occur when all extensions are disabled? Yes, I have launched vs code using`code --disable-extensions`, the issue remains the same. **I am looking for a way to disable this specific error as the stricter validation is probably intended. Ideally I do not want to change / rename the attributes beginning with "on".**