style attribute for the isprint tag

[matteobertoldo]

Describe the bug
When the <isprint> tag is defined and the attribute: style is added, attribute described in the official documentation, the highlight inside the attribute is the css language.
It must be replaced with the default color highlight of the string grammar, but only in this tag.

To Reproduce
Steps to reproduce the behavior:

1. Write <isprint> tag o create it with snippet
2. Set style attribute: <isprint style="style_ID">
3. See the highlight error as css language inside of this attribute

Expected behavior
The color highlight within the attribute must be replaced with the default string highlight.

Screenshots

Desktop

• OS: (macOS Catalina 10.15)
• Atom Version (1.42.0 x64)

Additional context
None.