Also define dependencies as peerDependencies

FIX: Declare package dependencies as peer dependencies as an attempt
to avoid duplicated package issues.

Issue #12
Issue codemirror/dev#858

package.json

@@ -31,6 +31,12 @@
     "@codemirror/view": "^6.0.0",
     "@lezer/common": "^1.0.0"
   },
+  "peerDependencies": {
+    "@codemirror/language": "^6.0.0",
+    "@codemirror/state": "^6.0.0",
+    "@codemirror/view": "^6.0.0",
+    "@lezer/common": "^1.0.0"
+  },
   "devDependencies": {
     "@codemirror/buildhelper": "^0.1.5"
   },