Syntax change: "extension on" instead of "extension of"

"extension on" is more precise. The thing after the "of/on" is a parameter.
The definition is not an extension "of" this parameter (what does that even
mean?), but rather an extension of the parameter's underlying type.

Note that Drat also uses "extension on".

Author: odersky

docs/docs/internals/syntax.md

@@ -103,7 +103,7 @@ yield
 ### Soft keywords
 
 ```
-derives   extension inline    opaque    open
+as        derives   extension inline    on       opaque    open
 ~         *         |         &         +         -
 ```
 

docs/docs/reference/contextual/extension-methods.md

@@ -131,19 +131,19 @@ A collective extension defines one or more concrete methods that have the same t
 and prefix parameter. Examples:
 
 ```scala
-extension stringOps of (xs: Seq[String]) {
+extension stringOps on (xs: Seq[String]) {
   def longestStrings: Seq[String] = {
     val maxLength = xs.map(_.length).max
     xs.filter(_.length == maxLength)
   }
 }
 
-extension listOps of [T](xs: List[T]) {
+extension listOps on [T](xs: List[T]) {
   def second = xs.tail.head
   def third: T = xs.tail.tail.head
 }
 
-extension of [T](xs: List[T]) with Ordering[T]) {
+extension on [T](xs: List[T]) with Ordering[T]) {
   def largest(n: Int) = xs.sorted.takeRight(n)
 }
 ```
@@ -178,7 +178,7 @@ DefSig            ::=  ...
 ExtParamClause    ::=  [DefTypeParamClause] ‘(’ DefParam ‘)’
 TmplDef           ::=  ...
                     |  ‘extension’ ExtensionDef
-ExtensionDef      ::=  [id] ‘of’ ExtParamClause {GivenParamClause} ‘with’ ExtMethods
+ExtensionDef      ::=  [id] ‘on’ ExtParamClause {GivenParamClause} ‘with’ ExtMethods
 ExtMethods        ::=  ‘{’ ‘def’ DefDef {semi ‘def’ DefDef} ‘}’
 ```