fix: improve prompting when deleting variables

[gonfunko]

The basics

• I validated my changes

The details

Resolves

This PR resolves several issues when deleting variables:

• Fixes a regression introduced in v12 where deleting a variable from the menu in the flyout would prompt to confirm that you wanted to delete the other blocks referencing that variable in the flyout
• Fixes Variable deletion confirmation prompt is not shown when deleting a variable with one reference from the flyout #8528, where deleting a variable from the menu in the flyout would not warn if only one block referencing that variable existed in the main workspace

Proposed Changes

Blockly.Variables.deleteVariable() now takes an optional third argument, triggeringBlock, corresponding to the block that triggered the deletion, which is used to filter it out of the list of block references correctly when determining whether to prompt the user. Before, it simply assumed that if there was one reference that was the block that triggered the deletion, but deletion across the flyout/workspace boundary violated that assumption. For backwards compatibility, this arg is optional, and if not provided (legacy apps, or programmatic variable deletion) the old "warn if > 1 references" behavior is preserved.