fix selection operations being duplicated (#2023)

#### Is this adding or improving a _feature_ or fixing a _bug_?

Bug.

#### What's the new behavior?

Fixes selection operations from being duplicated.

#### How does this change work?

Previously the selection properties were compared by reference, but paths are immutable `List` objects, which always show up as having changed, resulting in extra selection operations that without any real changes. We now use `Immutable.is` to remove those duplicates, fixing the undo history stack.

#### Have you checked that...?

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* [x] The new code matches the existing patterns and styles.
* [x] The tests pass with `yarn test`.
* [x] The linter passes with `yarn lint`. (Fix errors with `yarn prettier`.)
* [x] The relevant examples still work. (Run examples with `yarn watch`.)

#### Does this fix any issues or need any specific reviewers?

Fixes: #2006

packages/slate-react/src/plugins/after.js

@@ -347,6 +347,7 @@ function AfterPlugin() {
     const entire = selection
       .moveAnchorTo(point.key, start)
       .moveFocusTo(point.key, end)
+      .normalize(document)
 
     // Change the current value to have the leaf's text replaced.
     change.insertTextAtRange(entire, textContent, leaf.marks).select(corrected)

packages/slate-react/src/utils/find-range.js

@@ -60,7 +60,7 @@ function findRange(native, value) {
     }
   }
 
-  const range = Range.create({
+  let range = Range.create({
     anchorKey: anchor.key,
     anchorOffset: anchor.offset,
     focusKey: focus.key,
@@ -69,6 +69,7 @@ function findRange(native, value) {
     isFocused: true,
   })
 
+  range = range.normalize(value.document)
   return range
 }