Update changelog version following migration (#106)

Bump to version 1.0.0 and make changelog and pubspec agree.

Add changelog entry for the null safety migration.
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 881aa2f..832f0e7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,7 +1,7 @@
-# 0.9.8-dev
+# 1.0.0-nullsafety.0
 
+* Migrate to null safety.
 * Add the ability to create custom Watcher types for specific file paths.
-* Require at least Dart 2.8.4.
 
 # 0.9.7+15
 
diff --git a/pubspec.yaml b/pubspec.yaml
index 0309be5..d09361a 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,5 +1,5 @@
 name: watcher
-version: 0.10.0-nullsafety.0
+version: 1.0.0-nullsafety.0
 
 description: >-
   A file system watcher. It monitors changes to contents of directories and