Update textarea intrinsic height calculations
This change adds a new layout.forms.textarea-sizing-excludes-auto-scrollbar.enabled pref that controls the height calculations for textareas.
With the pref enabled (the default) textareas only include the scrollbar size in the height calculations when overflow-x is scroll, this matches Chromium and WebKit.
Also adds a corresponding WPT for this.
Differential Revision: https://phabricator.services.mozilla.com/D210796
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1830576
gecko-commit: 12723265fe8c5a7e4fe00b353a93bfc72276439d
gecko-reviewers: dholbert