Make prerelease version

Author: veloce

example/pubspec.lock

@@ -31,7 +31,7 @@ packages:
       path: ".."
       relative: true
     source: path
-    version: "7.0.0"
+    version: "7.0.0-beta.1"
   clock:
     dependency: transitive
     description:

pubspec.yaml

@@ -1,6 +1,6 @@
 name: chessground
 description: Chess board UI developed for lichess.org. It has no chess logic inside so it can be used for chess variants.
-version: 7.0.0
+version: 7.0.0-beta.1
 repository: https://github.com/lichess-org/flutter-chessground
 funding:
   - https://lichess.org/patron