Treat whitespace-only text nodes between element siblings as part of the
previous element instead of creating a separate text node.
generateFragmentFromHtml now detects when a text node consisting only of
space appears between two elements and appends that space to the
previous instance's last text child (if present), avoiding creation of a
standalone text child. Added a regression test in plugin-html.test.tsx
to verify pasted HTML like `<span>✓</span> <span>text</span>` preserves
the space and results in two span instances (no separate text node),
ensuring text-content control remains correct in the settings panel.
Ref https://github.com/webstudio-is/webstudio/issues/2651
There is an edge case which will prevent from converting breakpoints.
When there is no base breakpoint there is no value to fill with inverted
space. Though it should be very rare usecase. Need to test how often it
will create new breakpoints.