So that the addon doesn't try to put a single DOM node in two places

Closes #4848

Add tag parameter to xml-fold findEnclosingTag to find enclosing tag of particular type.

Closes #4548

Closes #4018

Closes #4056

Use findMarks instead of findMarksAt in foldGutter.js to correctly
find marks when a single line is collapsed so that the arrow displays
correctly as collapsed.

Add option 'clearOnEnter' to foldcode.js and default it to true.
This option determines whether the marker added when folding a
line clears when the cursor enters it.

Since the viewport info isn't updated until the redraw, and the event
fires before the redraw.

See https://discuss.codemirror.net/t/whats-wrong-fold-gutters-do-not-show-up-after-swapdoc-call/580

Closes #3651

Closes #2581

This way, the matchtags addon highlights them the same as normal tags.

Closes #2737

Using the CodeMirror-guttermarker and CodeMirror-guttermarker-subtle CSS
classes to give the markers their color.

Issue #2602

Bumping them for every release just adds too much git
noise and history blowup.

Issue #2557

So that the click doesn't kill editor focus.

Closes #2505

Issue #2253

Add getHelpers and registerGlobalHelper, allow modes to specify arrays
of helper names, support modeProps in mode specifications.

Update foldcode and show-hint to make use of more powerful helper
system by defining an `auto` helper that tries multiple helpers, and
using that as default.

Issue #2025

Issue #1859

Fix next line after block being incorrectly folded into the block
Fix detection of the end of a code block

Issue #1865

Issue #1739