(core) Trim trailing whitespace when pasting plain text

Summary: title

Test Plan:
Tested manually. Blank rows at the end are no longer pasted. Pasting multiple columns separated by tabs can still have blank cells in some cells of the final rows.

I don't think this needs an automated test.

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D3110

app/client/components/Clipboard.js

@@ -154,7 +154,7 @@ Clipboard.prototype._onPaste = function(elem, event) {
     if (plainText === '' || plainText.charCodeAt(0) === 0xFEFF) {
       data = [['']];
     } else {
-      data = tsvDecode(plainText.replace(/\r\n?/g, "\n"));
+      data = tsvDecode(plainText.replace(/\r\n?/g, "\n").trimEnd());
     }
   }