Add Node.getIf method

[thesunny]

Problem
If we need to check if a node exists at a given path and return it, we currently have to do this in several steps: (1) check to see if the Node exists using Node.has. (2) check the return value and if it's false we end and if it's true we (3) call Node.get. We need to do it in this order because if we call Node.get and the path doesn't exist, Slate will throw an error.

The code for these two methods are nearly identical and both require us to loop through the Document to get to the path.

Solution
Instead, we can just add as a method Node.getIf which returns the Node if it exists or undefined if it does not exist.

This has the performance benefit of not iterating through the DOM twice and also simplifies usage of Slate.

A benefit of this is that we can have Node.get and Node.has call Node.getIf and the addition of this feature may actually result in a net decrease of code.

Alternatives
We can do a try/catch with Node.get but this complicates the code rather than simplifies it.

Context
I think this is a common use case. For example, if we want to find out what Node is before or after the current node, we would be currently using Node.has followed by Node.get passing in the path of Path.previous or Path.next. Adding this method means we only have to use Node.getIf.

[LeonSo7]

+1