libgit2 API:
https://libgit2.org/docs/reference/main/worktree/index.html

I had a use case for git worktrees here:
DavisVaughan/cross#8

Basically I wanted to be on my-branch and be able to:

• Check out main, install the package at main into a specific libpath
• Check out my-branch, install the package at my-branch into a specific libpath

All while not touching the user's current state while on my-branch

My solution was to:

• Create a temp branch, main-cross
• Create a worktree in a temp dir, checkout out main-cross there
• Create a temp branch, my-branch-cross
• Create a worktree in a temp dir, checkout out my-branch-cross there

This seems like the right way to do this, but it requires me to go to system git calls. I copied some code from pak that does this, but I'm already using gert so it would be nice to just stay in that ecosystem.

I think minimal support for me would look like:

git_worktree_add <- function(dir, branch) {
}

git_worktree_remove <- function(dir) {
}