Fix bug: setting selection from contentEditable:false element causes crash

Fixes https://github.com/ianstormtaylor/slate/issues/4583

When clicking some text in a `contentEditable:false` element, if the
handler for this sets the selection, Slate crashes. Slate tries to find
a Slate range for the text that was clicked on, but there is no such
range, because the text is inside a `contentEditable:false` element.
Slate seems to be making a bad assumption that the current DOM selection
necessarily corresponds to a Slate range, but this is not the case if
the user just clicked into an element with `contentEditable: false`.
To fix this, I changed `exactMatch: false` to `exactMatch: true`,
which seems to mean "fail gracefully if there is no exact match".

packages/slate-react/src/components/editable.tsx

@@ -182,7 +182,9 @@ export const Editable = (props: EditableProps) => {
     // If the DOM selection is in the editor and the editor selection is already correct, we're done.
     if (hasDomSelection && hasDomSelectionInEditor && selection) {
       const slateRange = ReactEditor.toSlateRange(editor, domSelection, {
-        exactMatch: false,
+        // domSelection is not necessarily a valid Slate range
+        // (e.g. when clicking on contentEditable:false element)
+        exactMatch: true
       })
       if (slateRange && Range.equals(slateRange, selection)) {
         return