Update PlainBasePart to only split on space boundaries

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

src/editor/parts.ts

@@ -186,7 +186,11 @@ abstract class PlainBasePart extends BasePart {
         }
         // when not pasting or dropping text, reject characters that should start a pill candidate
         if (inputType !== "insertFromPaste" && inputType !== "insertFromDrop") {
-            return chr !== "@" && chr !== "#" && chr !== ":";
+            if (chr !== "@" && chr !== "#" && chr !== ":") {
+                return true;
+            }
+            // only split if the previous character is a space
+            return this._text[offset - 1] !== " ";
         }
         return true;
     }