The `}` and `{` paragraph motions now correctly treat only truly empty
lines (zero characters) as paragraph boundaries, matching vim's
documented behavior. Whitespace-only lines are no longer treated as
boundaries.

Changed `start_of_paragraph()` and `end_of_paragraph()` in
`editor/src/movement.rs` to check `line_len() == 0` instead of
`is_line_blank()`.

Note: This change does NOT affect the `ap`/`ip` text objects. Per vim's
`:help ap`, those DO treat whitespace-only lines as boundaries, which is
the existing (correct) behavior in `vim/src/object.rs`.

Closes #36171

Release Notes:

- Fixed vim mode paragraph motions (`}` and `{`) to correctly ignore
whitespace-only lines

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Co-authored-by: dino <dinojoaocosta@gmail.com>