Add functionality to escape values in selectors

[SteffenDE]

Feature goal

In the browser, there is the CSS.escape function that is very useful when constructing CSS selectors. It would be great if Floki had a similar functionality. Instead of Floki.find(html, "##{id} .foo") we could write Floki.find(html, "##{Floki.css_escape(id)} .foo"). This could be especially useful in LiveView tests.

Relates to: #551
Relates to: phoenixframework/phoenix_live_view#3159
Relates to: phoenixframework/phoenix_live_view#3145

iex> Floki.css_escape("abc?")
"abc\\?"
iex> Floki.css_escape("abc[]")
"abc\\[\\]"

Dependencies

A functionality like this would require Floki to be able to properly parse the escaped values. I'm not sure if this is currently always the case.