In this commit, we rename the files as assembler.go houses the primary
interfaces/abstractions of the package. In the rest of the codebase,
this file is near uniformly called interface.go, so we rename the file
to make the repo more digestible at a scan.

In this commit, we consolidate the root bucket TLVs into a new struct.
This makes it easier to see all the new TLV fields at a glance. We also
convert TLV usage to use the new type param based APis.

This'll allow us to create a funding output that uses musig2, but uses a tapscript tweak rather than a normal BIP 86 tweak.

In most cases, we won't yet be passing a root. The option usage helps us keep the control flow mostly unchanged.

This isn't hooked up yet to the funding manager, but with this commit, we can now start to write internal unit tests that handle musig2 channels with a tapscript root.

With this commit, the channel is now aware of if it's a musig2 channel, that also has a tapscript root. We'll need to always pass in the tapscript root each time we: make the funding output, sign a new state, and also verify a new state.