[Feature Request] Refactor rename for existing environment variables

- Insomnia Version: 7.0.3
- Operating System: Ubuntu 18.04.3 LTS

## Details

When I try to rename environment variables, all request that use old variable show error "attempted to output null or undefined value".
so I also need to check each request and rename its variable. This process (refactor rename) could be automate in the same way IDE try to refactor rename a variable by find usages and applying rename.
I think its relate to #1001 but I'm not sure that this is the same issue or can be solved using same solution.

My proposed solution for this is adding the feature when user click the variable in one request so Edit Variable modal window is open and add some button that function as refactor rename.

Start by reproducing the environment-variable rename flow in Insomnia and inspect the Edit Variable modal mentioned in the issue. Review related issue #1001 and trace requests that reference the renamed variable. Done means renaming updates all usages without the "attempted to output null or undefined value" error.

Written by the indexing model from the issue text.