Add Directory Go Into node

[RunDevelopment]

Closes #2519
Closes #606

This adds the last node from #2519. As before I made a few changes:

• I named it "Directory Go Into" to be consistent with the "Directory Go Up" node.
• I added a few optional folder inputs, so users can join multiple folders without needing to know that / and \ are usually used to denote this.
• More path validation: no invalid characters and absolute paths.

The output type of this node is kind of insane because Navi doesn't have early returns.

[joeyballentine]

LGTM. Damn the nesting on the Navi type is so gross