Provide syntax checking for `cssRule`

[okarmazin]

If you declare a raw CSS selector containing parentheses, such as nth-child(even), without the leading colon, website output will be broken (I have no better word to describe this). Omitting the colon on selectors without parens (hover) doesn't lead to breakages.

Very easily reproducible:

1. kobweb create app
2. Replace the HomeGridCellStyle in Index.kt with the following block.
3. Start the dev server and observe the differences in website behavior when adding and removing the colon in nth-child

val HomeGridCellStyle = CssStyle {
    base {
        Modifier
            .backgroundColor(GridCellColorVar.value())
            .boxShadow(blurRadius = 0.6.cssRem, color = GridCellColorVar.value())
            .borderRadius(1.cssRem)
    }
    cssRule("nth-child(even)") {
        Modifier.backgroundColor(org.jetbrains.compose.web.css.Color.chartreuse)
    }
    cssRule(":hover") {
        Modifier.backgroundColor(org.jetbrains.compose.web.css.Color.crimson)
    }
}

Version: 0.19.0

[bitspittle]

This is not a bug with Kobweb. You have to know what you're doing when using cssRule.

That said, maybe we can explore doing syntax checking in the Kobweb IntelliJ Plugin.