Add >>, the indent-region operator

[antifuchs]

This operator inserts all elements as normal, and then updates their indentation to match whatever indent-region would do. This helps get correctly-indented text in modes where unbalanced structure confuses the indentation engine.

The whole thing was discussed and proposed in #160 (reply in thread)

The change isn't super straightforward, as we need a bit more logic to support placeholder fields moving:

• tempel--inhibit-clearing-placeholders is a new flag that allows the normal field-changing hooks to run, but prevents the default from changing, if indentation changed the field.
• A new branch on the after case of tempel--field-modified that moves the start of the overlay if text should be inserted before it (as indentation does).