TypeScript '-=' subtraction assignment operator not supported

(really an improvement more than a bug)

When code is decompiled back to blocks by the emitter the '-=' operator is not supported,
e.g.

material.roughness -= myroughness

becomes,

Similarly using a negative operand with the '+=' operator we get an interesting round-trip,

material.roughness += -myroughness

becomes,

and then when switching back to code,

material.roughness += 0 - myroughness

If we introduced a negative block that would return negated numerical input we could special case this block in conjunction with the change block as its parent to compile/decompile to/from the '-=' operator and its associated expression.