Preserve leading spaces in code

What does this MR do and why?

Preserve leading spaces in code

In RTE, when coming from a plain text editor, if an inline code block contains leading spaces, preserve them. Also make sure they are preserved after serialization.

Changelog: fixed

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Before	After

How to set up and validate locally

1. In a text box, using plain text editor, type in an inline code block with leading spaces. Example: ` code`.
2. Switch to RTE. The leading spaces should be preserved.
3. Add some text to RTE.
4. Switch back to plain text editor. The leading spaces should still be preserved.

Related to #442106 (closed)