Html Reader Preserve Unicode Whitespace Characters

[oleibman]

Fix #1284, which was closed as stale in 2019, but which I will now reopen. Html Reader converts Unicode whitespace characters in a DOM text node to space. However, Html treats only space, tab, CR, LF, vertical tab, and form-feed as whitespace. Using a regular expression with the u (Unicode) modifier causes a number of other characters to be converted to space inappropriately. The issue mentions "ideographic space" in particular, stating that it is used for formatting and should be preserved. "Non-breaking space" is also used in the same way and should also be preserved. An exception is made for a text node consisting of a single non-breaking space, since that is used as a placeholder by Html Writer; my own guess is that this is the reason why the Unicode modifier was used in the first place.

This is:

• a bugfix
• a new feature
• refactoring
• additional unit tests

Checklist:

• Changes are covered by unit tests
  • Changes are covered by existing unit tests
  • New unit tests have been added
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change and a link to the pull request if applicable
• Documentation is updated as necessary

Why this change is needed?

Provide an explanation of why this change is needed, with links to any Issues (if appropriate).
If this is a bugfix or a new feature, and there are no existing Issues, then please also create an issue that will make it easier to track progress with this PR.