[Feature Request] Triangle linear counter retrigger command

[Gumball2415]

Works similar to DPCM retrigger command (Xxx), but the xx parameter will control linear counter duration instead of tick period.

This command will write the linear counter value xx on every tick the command is active for.

Requested by @zeta0134

[zeta0134]

Proposed documentation:

Triangle Effects
----------------

**Xxx - Linear Trill**

Retrigger's the linear counter on each tick, with a period of **xx** in APU Quarter Frames. Use xx=00 to disable.
Valid range: 00 - 7F

I'm not emotionally attached to Xxx, but as it performs a similar automatic retriggering for DPCM sample playback, it seems to fit.

[zeta0134]

Motivation: this setup is used to great effect in Namco's Pacman CE demake for NES, here:

https://youtu.be/Fg8B3kiVckY?t=60

And here:

https://youtu.be/zAmF4QRf8eE?t=16

It is currently somewhat possible to recreate this effect in FamiTracker by using speed = 1, but this is both irritating to compose against and very inefficient in terms of the compiled bytecode data. The effect is pretty simple to implement in a driver, so it makes sense to include it as an automation.

[Gumball2415]

this feature will be queued after #262