tocss: Simplify the hugo:vars type handling

tocss: Simplify the hugo:vars type handling

Instead of maintaing a list of all CSS units and functions this commit:

* Uses 3 regexps to detect typed CSS values (e.g. `24px`) + properly handle numeric Go types.
* These regexps may have some false positives -- e.g. strings that needs to be quoted.
* For that rare case, you can mark the string with e.g. `"32xxx" | css.Quoted`
* For the opposite case:  `"32" | css.Unquoted`

Updates #10632