feat: `vm.depth()` cheatcode to return the depth of the current invariant run

[mds1]

Component

Forge

Describe the feature you would like

Two enables two use cases that I can't think of good workarounds for:

1. Logging all sequences for all runs to a file with vm.writeLine()
2. Skipping checks on depth=0, i.e. skip the "invariant must hold at the start" check. The use case here is testing that a variable monotonically increases and before any calls are made, since at the start "last value" and "current value" are the same.

Additional context

No response