support maintaining multiple versions/branches on main

[davidfirst]

In some scenarios, it's needed to maintain some legacy code, for example, the new code is 2.x and the legacy is 1.x which still needs to be maintained.
If a patch is needed to 1.x code, the component needs to be checked out to 1.x and tagged with a specific version, for example: bit tag --ver 1.0.1 --ignore-newest-version.
Currently, when this patch is done, although the history of this patch is correct thanks to a recent PR, but the head now points to this patch version. As a result, when importing the component, the user gets 1.0.1 and not 2.x.
This PR introduces a new flag --detach-head to keep the head intact in this case, so the head still points to 2.x, while the .bitmap is updated to 1.0.1. This new patch 1.0.1 becomes a detached head.

It also supports lanes. It's possible to work on multiple components when they're checked out to 1.x, create a new lane, snap some changes, then merge the lane with --detach-head flag. This flag, instruct bit to find the common snap before the divergent point, and merge the lane to that point and not to the head.

In order for the garbage collector to not delete this detached-head snap, and for the remote to not identify this component as a merge-conflict, we save a new prop detachedHeads into ModelComponent object.

For now, this flag --detach-head is under a feature-flag. It'll be enabled by default once bit cloud is deployed with a new version that supports it.