feat: add FocusManager

This is the bulk of the work for introducing the central logical unit
for managing and sychronizing focus as a first-class Blockly concept
with that of DOM focus.

There's a lot to do yet, including:
- Ensuring clicks within Blockly's scope correctly sync back to focus
  changes.
- Adding support for, and testing, cases when focus is lost from all
  registered trees.
- Testing nested tree propagation.
- Testing the traverser utility class.
- Adding implementations for IFocusableTree and IFocusableNode
  throughout Blockly.

core/css.ts

@@ -484,4 +484,13 @@ input[type=number] {
 .blocklyDragging .blocklyIconGroup {
   cursor: grabbing;
 }
+
+.blocklyActiveFocus {
+  outline-color: #2ae;
+  outline-width: 2px;
+}
+.blocklyPassiveFocus {
+  outline-color: #3fdfff;
+  outline-width: 1.5px;
+}
 `;