Hide trailing whitespace when hiding markup

[jimporter]

Description

When hiding markup, e.g. via markdown-view-mode, the trailing whitespace markup for line breaks doesn't get hidden. This patch changes it so that the whitespace is hidden when hiding other markup. In practice, this just makes the " " invisible and nothing else; then, we let the newline do its usual thing.

(In the future, I could imagine markdown-view-mode handling soft newlines differently, so that something like foo\nbar is displayed as foo bar. Then, we'd probably want to do something special to mark the newline in \n as a hard newline.)

Related Issue

See https://debbugs.gnu.org/cgi/bugreport.cgi?bug=69647

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• Improvement (non-breaking change which improves an existing feature)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• I have read the CONTRIBUTING.md document.
• I have updated the documentation in the README.md file if necessary.
• I have added an entry to CHANGES.md.
• I have added tests to cover my changes.
• All new and existing tests passed (using make test).

[syohex]

Thanks