represent VCS forks and branches in Biome’s versioning model

[tayloraswift]

branches are crucial because we want to support updating documentation for multiple branch heads, not just master@latest.

instead of having a contiguous metadata buffer for a package’s entire history, we should have contiguous buffers for each branch, which (except for master/main) refer to some parent branch up to a fork point. branches should only store information that is not in the parent branch slice, to reduce data duplication.

if we overwrite a node in a branch, only the descendants of that branch get disturbed, instead of the entire package.