#### Add basic support for macro annotations
* Introduce experimental `scala.annotations.MacroAnnotation`
* Macro annotations can analyze or modify definitions
* Macro annotation can add definition around the annotated definition
 * Added members are not visible while typing
 * Added members are not visible to other macro annotations
 * Added definition must have the same owner
* Implement macro annotation expansion
 * Implemented at `Inlining` phase 
* Can use macro annotations in staged expressions (expanded when at
stage 0)
 * Can use staged expression to implement macro annotations 
 * Can insert calls to inline methods in macro annotations
 * Current limitations (to be loosened in following PRs)
 * Can only be used on `def`, `val`, `lazy val` and `var`
 * Can only add `def`, `val`, `lazy val` and `var` definitions
#### Example
```scala
class memoize extends MacroAnnotation:
 def transform(using Quotes)(tree: quotes.reflect.Definition): List[quotes.reflect.Definition] =
 import quotes.reflect._
 tree match
 case DefDef(name, TermParamClause(param :: Nil) :: Nil, tpt, Some(rhsTree)) =>
 (Ref(param.symbol).asExpr, rhsTree.asExpr) match
 case ('{ $paramRefExpr: t }, '{ $rhsExpr: u }) =>
 val cacheTpe = TypeRepr.of[Map[t, u]]
 val cacheSymbol = 
 Symbol.newVal(tree.symbol.owner, name + "Cache", cacheTpe, Flags.Private, Symbol.noSymbol)
 val cacheRhs = '{ Map.empty[t, u] }.asTerm
 val cacheVal = ValDef(cacheSymbol, Some(cacheRhs))
 val cacheRefExpr = Ref(cacheSymbol).asExprOf[Map[t, u]]
 val newRhs = '{ $cacheRefExpr.getOrElseUpdate($paramRefExpr, $rhsExpr) }.asTerm
 val newTree = DefDef.copy(tree)(name, TermParamClause(param :: Nil) :: Nil, tpt, Some(newRhs))
 List(cacheVal, newTree)
 case _ =>
 report.error("Annotation only supported on `def` with a single argument are supported")
 List(tree)
```
with this macro annotation a user can write
```scala
@memoize
def fib(n: Int): Int =
 println(s"compute fib of $n")
 if n <= 1 then n else fib(n - 1) + fib(n - 2)
```
and the macro will modify the definition to create
```scala
val fibCache = mutable.Map.empty[Int, Int]
def fib(n: Int): Int =
 fibCache.getOrElseUpdate(
 n,
 {
 println(s"compute fib of $n")
 if n <= 1 then n else fib(n - 1) + fib(n - 2)
 }
 )
```
#### Based on
* #15626
* https://infoscience.epfl.ch/record/294615?ln=en
#### Followed by
* #16454 

Enable the addition of classes from a `MacroAnnotation`:
 * Can add new `class` definitions next to the annotated definition
Special cases:
 * A top-level `def`, `val`, `var`, `lazy val` can return a `class`
 definition that is owned by the package or package object.
Related PRs:
 * Follows scala#16454 

Enable the addition of classes from a `MacroAnnotation`:
 * Can add new `class` definitions next to the annotated definition
Special cases:
 * An annotated top-level `def`, `val`, `var`, `lazy val` can return a `class`
 definition that is owned by the package or package object.
Related PRs:
 * Follows scala#16454 

Enable the addition of classes from a `MacroAnnotation`:
 * Can add new `class` definitions next to the annotated definition
Special cases:
 * An annotated top-level `def`, `val`, `var`, `lazy val` can return a `class`
 definition that is owned by the package or package object.
Related PRs:
 * Follows scala#16454 

Enable the addition of classes from a `MacroAnnotation`:
* Can add new `class`/`object` definitions next to the annotated
definition
Special cases:
* An annotated top-level `def`, `val`, `var`, `lazy val` can return a
`class`/`object`
 definition that is owned by the package or package object.
Related PRs:
 * Follows #16454 

Enable the addition of classes from a `MacroAnnotation`:
 * Can add new `class` definitions next to the annotated definition
Special cases:
 * An annotated top-level `def`, `val`, `var`, `lazy val` can return a `class`
 definition that is owned by the package or package object.
Related PRs:
 * Follows scala#16454