Remove CSSVariables context

One can define CSS variables anywhere
This is actually  the only thing we want to do right now in framework
of #894

Resolves #894

web/benchmark-core/src/jsMain/kotlin/com/sample/content/CodeSamplesSwitcher.kt

@@ -7,7 +7,7 @@ import org.jetbrains.compose.web.attributes.*
 import org.jetbrains.compose.web.dom.*
 import com.sample.style.AppStylesheet
 
-private object SwitcherVariables : CSSVariables {
+private object SwitcherVariables {
     val labelWidth by variable<CSSpxValue>()
     val labelPadding by variable<CSSpxValue>()
 }

web/core/src/jsMain/kotlin/org/jetbrains/compose/web/css/StyleBuilder.kt

@@ -52,8 +52,6 @@ fun StyleBuilder.add(
     value: StylePropertyValue
 ) = property(propertyName, value)
 
-interface CSSVariables
-
 interface CSSVariable {
     val name: String
 }
@@ -79,7 +77,7 @@ fun <TValue> CSSStyleVariable<TValue>.value(fallback: TValue? = null)
         )
     )
 
-fun <TValue: StylePropertyValue> CSSVariables.variable() =
+fun <TValue: StylePropertyValue> variable() =
     ReadOnlyProperty<Any?, CSSStyleVariable<TValue>> { _, property ->
         CSSStyleVariable(property.name)
     }

web/core/src/jsTest/kotlin/CSSStylesheetTests.kt

@@ -13,7 +13,7 @@ import org.w3c.dom.get
 import kotlin.test.Test
 import kotlin.test.assertEquals
 
-object AppCSSVariables : CSSVariables {
+object AppCSSVariables {
     val width by variable<CSSUnitValue>()
     val height by variable<CSSUnitValue>()
 

web/core/src/jsTest/kotlin/CSSUnitApiTests.kt

@@ -491,7 +491,7 @@ class CSSUnitApiTests {
 
     @Test
     fun calcVaraiables() {
-        val variables = object : CSSVariables {
+        val variables = object {
             val pxVar by variable<CSSSizeValue<CSSUnit.px>>()
         }
         val typedResultLength: CSSNumericValue<CSSUnitLength> = 4.pt + variables.pxVar.value()