refactor: replace deprecated String.prototype.substr()

.substr() is deprecated so we replace it with .slice() which works similarily but isn't deprecated

Signed-off-by: Tobias Speicher <rootcommander@gmail.com>

src/browser/Terminal.test.ts

@@ -1332,8 +1332,8 @@ describe('Terminal', () => {
           (!(i % 3))
             ? input[i]
             : (i % 3 === 1)
-              ? input.substr(i, 2)
-              : input.substr(i - 1, 2),
+              ? input.slice(i, i + 2)
+              : input.slice(i - 1, i + 1),
           terminal.buffer.lines.get(bufferIndex[0])!.loadCell(bufferIndex[1], new CellData()).getChars());
       }
     });