make consecutive :/@/# separate pill candidates

e.g. accept the first character always to start the autocompletion
but after that refer to the plainpart logic to split up in
new pill candidates. Also, don't allow merging adjacent parts.

This makes sure that "@nonmatchingfoo @someuser"
only matches "someuser" in the autocomplete.

src/editor/parts.js

@@ -313,8 +313,16 @@ export class PillCandidatePart extends PlainPart {
         return this._autoCompleteCreator(updateCallback);
     }
 
-    acceptsInsertion(chr) {
-        return true;
+    acceptsInsertion(chr, i) {
+        if (i === 0) {
+            return true;
+        } else {
+            return super.acceptsInsertion(chr, i);
+        }
+    }
+
+    merge() {
+        return false;
     }
 
     acceptsRemoval(position, chr) {