Update BlockDefinition type import in block definition files

[BeksOmega]

Describe the bug

Currently, because we are in the middle of the typescript migration, we cannot use const {BlockDefinition} = goog.requireType('Blockly.blocks'); in the block definition files, because BlockDefinition is a type, which gets stripped when TypeScript transpiles to JS. So it doesn't exist when the closure module system goes to try to find it.

When we turn the block definitions into typescript/esmodules, we should try to properly import the BlockDefinition type using import statements.

[cpcallen]

Too soon, GitHub, too sooon.