[lexical-plaintext] Feature: add escape key handler

[jeffreyclu]

Description

Adds the same escape key handler to the plain text package as exists in lexical/richtext.

Closes: #5922

Test plan

Before

In the plaintext playground, pressing Esc doesn't blur the editor.

https://github.com/facebook/lexical/assets/55211974/a34cdd6e-d0a4-483e-ae46-f80759b39340

After

In the plaintext playground, pressing Esc blurs the editor.

https://github.com/facebook/lexical/assets/55211974/3a219753-03c5-4c92-9eeb-d44ce421d112