Local variables should all be sorted after global variables

Expected Behavior

Local variables should, in the block palette, be sorted so that they all appear (in alphanumeric order) after all the global variables.

Actual Behavior

Local variables and global variables are sorted together.

Steps to Reproduce

Create a global variable such as "A". Create a local variable such as "B". Create a global variable such as "C". Note that the sort order is A, B (local), C, not A, C, B (local).

This sort of ties into #1379. I believe in 2.0 all variables are sorted together, so I suppose this is a request more than a compatibility issue. It should be noted that Scratch 1.4 has a visible horizontal-rule separator between global variables and local variables:

I don't think scratch-blocks has support for <hr>-style separators, but it does have ordinary separator spaces (e.g. between "point towards mouse-pointer" and "change x by 10"). Could be useful here.

Start by tracing how the block palette collects and sorts global and local variables, then reproduce the A/global, B/local, C/global example from the issue. Done means globals appear first in alphanumeric order, followed by locals in alphanumeric order, with the resulting palette behavior matching the requested ordering.