CSS Tidy: Add `user-select` support

### Steps to Reproduce

Add this CSS (to prevent text-selection on sites) on Additional CSS:

```
body {
-webkit-touch-callout: none; /* iOS Safari */
-webkit-user-select: none; /* Safari */
-khtml-user-select: none; /* Konqueror HTML */
-moz-user-select: none; /* Old versions of Firefox */
-ms-user-select: none; /* Internet Explorer/Edge */
user-select: none; /* Non-prefixed version, currently supported by Chrome, Opera and Firefox */
}
```

That CSS is to prevent text selection. The CSS code is from this guide:
https://www.wpbeginner.com/wp-tutorials/how-to-prevent-text-selection-and-copy-paste-in-wordpress-easy/

### What I expected to happen

The CSS to work and for the users to be able to protect their contents

### What actually happened

CSS get stripped out. It seems like the CSS selectors there are not supported

### Context

User report:
4276268-zd-woothemes

Slack discussion:
p1630917680387500-slack-C03TY6J1A
p1630918555002300-slack-C029WGXE5T4

### Operating System

Windows, macOS, Linux, Android, iOS

### OS Version

_No response_

### Browser

Chrome/Chromium, Firefox, Microsoft Edge, Microsoft Edge (legacy), Safari

### Browser Version(s)

_No response_

### Is this specific to applied theme? If so, what is the theme name?

_No response_

### Simple/Atomic

Simple

### Console and/or error logs

_No response_

### Number of Users Impacted

_No response_

### Available Workarounds

None

### Reproducibility

_No response_

### Other information

_No response_

Reproduce the issue by adding the provided declarations to WordPress Additional CSS and observe which rules CSS Tidy strips. Trace the CSS Tidy sanitization entry point, then verify that the listed user-select declarations are retained and that the Additional CSS reproduces the intended behavior.

Written by the indexing model from the issue text.