Modulation FX LFO stereo updates

[Andreya-Autumn]

The Chorus, Flanger and Phaser share the modulator between them. In Surge today, only the phaser has a stereo control.

First of all, It'd make sense to give this stereo control to the flanger and phaser too.

Also, it doesn't make any meaningful difference for the noise and S&H shapes. There are ways to make it do that, which would be nice to add eventually. The latter would be a breaking change in Surge so would need streaming there, but we could also just not bother until XT2.