Add IME preedit ranges

[Riateche]

Added SetPreedit action that inserts text with a special is_preedit flag in the attributes and clears any previous preedit. This is the most straightforward way to show a preedit inline, so that it works naturally with shaping and word wrapping. The downside is that the buffer's text is not the same as the document text anymore. For example, if the user wants to retrieve the text from the buffer, they should delete the part corresponding to the preedit (or clear the preedit). To help with this, I added Buffer::text_without_preedit() and BufferLine::text_without_preedit().

I've added IME support to the editor example to demonstrate the correct handling of IME.

[Riateche]

Rebased onto main and fixed a CI issue with no_std. Please take a look.