Enable significant indentation after if.

There's one scenario where this breaks:

```
if
  (...
   ...)
  println()
else if ...
```
This will be interpreted as
```
if {
  (...
   ...)
  println()
}
else if ...
```
and will fail with `then` expected at the point of the `else`.

An example like this occurred in `SymDenotations.scala`.

Author: odersky

compiler/src/dotty/tools/dotc/core/SymDenotations.scala

@@ -832,11 +832,12 @@ object SymDenotations {
                | Access to protected $this not permitted because enclosing ${ctx.owner.enclosingClass.showLocated}
                | is not a subclass of ${owner.showLocated} where target is defined""")
         else if
-          (!(  isType // allow accesses to types from arbitrary subclasses fixes #4737
+          !(  isType // allow accesses to types from arbitrary subclasses fixes #4737
            || pre.derivesFrom(cls)
            || isConstructor
            || owner.is(ModuleClass) // don't perform this check for static members
-           ))
+           )
+        then
           fail(
             i"""
                | Access to protected ${symbol.show} not permitted because prefix type ${pre.widen.show}

compiler/src/dotty/tools/dotc/parsing/Tokens.scala

@@ -270,7 +270,7 @@ object Tokens extends TokensCommon {
   final val closingRegionTokens = BitSet(RBRACE, CASE) | statCtdTokens
 
   final val canStartIndentTokens: BitSet =
-    statCtdTokens | BitSet(COLONEOL, EQUALS, ARROW, LARROW, WHILE, TRY, FOR, RETURN)
+    statCtdTokens | BitSet(COLONEOL, EQUALS, ARROW, LARROW, WHILE, TRY, FOR, RETURN, IF)
       // `if` is excluded because it often comes after `else` which makes for awkward indentation rules  TODO: try to do without the exception
 
   /** Faced with the choice between a type and a formal parameter, the following

docs/docs/reference/other-new-features/indentation.md

@@ -28,7 +28,7 @@ There are two rules:
        class, object, given, or enum definition, in an enum case, or after a package clause, or
      - after one of the following tokens:
     ```
-    =  =>  <-  then  else  while  do  try  catch  finally  for  yield  match  return
+    =  =>  <-  if  then  else  while  do  try  catch  finally  for  yield  match  return
     ```
     If an `<indent>` is inserted, the indentation width of the token on the next line
     is pushed onto `IW`, which makes it the new current indentation width.