Allow new import syntax

[odersky]

• Replace wildcard import _ with *, which is what basically all other languages use
• Replace renaming => with a soft keyword as.
• Allow as outside braces, as in

  import scala.collection.mutable as mut
  import NumPy as np

From the doc page:

The syntax of wildcard and renaming imports (and exports) has changed.

Wildcard Imports

Wildcard imports are now expressed with * instead of underscore. Example:

import scala.annotation.*  // imports everything in the annotation package

If you want to import a member named * specifically, you can use backticks around it.

object A:
   def * = ...
   def min = ...

object B:
  import A.`*`   // imports just `*`

object C:
  import A.*     // imports everything in A

Renaming Imports

To rename or exclude an import, we now use as instead of =>. A single renaming import no longer needs to be enclosed in braces. Examples:

import A.{min as minimum, `*` as multiply}
import Predef.{augmentString as _, *}     // imports everything except augmentString
import scala.annotation as ann
import java as j

Migration

To support cross-building, Scala 3.0 supports the old import syntax with _ for wildcards and => for renamings in addition to the new one. The old syntax
will be dropped in a future versions. Automatic rewritings from old to new syntax
are offered under settings -source 3.1-migration -rewrite.

Syntax

Import            ::=  ‘import’ ImportExpr {‘,’ ImportExpr}
Export            ::=  ‘export’ ImportExpr {‘,’ ImportExpr}
ImportExpr        ::=  SimpleRef {‘.’ id} ‘.’ ImportSpec
ImportSpec        ::=  NamedSelector
                    |  WildcardSelector
                    | ‘{’ ImportSelectors) ‘}’
NamedSelector     ::=  id [‘as’ (id | ‘_’)]
WildCardSelector  ::=  ‘*' | ‘given’ [InfixType]
ImportSelectors   ::=  NamedSelector [‘,’ ImportSelectors]
                    |  WildCardSelector {‘,’ WildCardSelector}

[odersky]

Based on #11240

[odersky]

@liufengyun Can you review the technical side here? It looks like the proposal has strong support, so we will probably go ahead and merge this.

[liufengyun]

LGTM