Feature: support for custom environments in shared environment variables

[TowardsDeath]

Description

Similar to #245, I'd like to assign assign custom environments to a shared environment variable. This is currently possible via the UI, but not with the Terraform provider.

Current Behavior

The resource vercel_shared_environment_variable currently only allows the default environments provided by Vercel in its target field

Valid targets are either production, preview, or development.

https://registry.terraform.io/providers/vercel/vercel/latest/docs/resources/shared_environment_variable#target-1

Expected Behavior

Ideally it should work like the UI: the target field of the resource vercel_shared_environment_variable should accept a value "all_custom_environments" (or something similar).

Example Usage

resource "vercel_shared_environment_variable" "example" {
  key            = "EXAMPLE"
  value          = "some_value"
  target         = "all_custom_environments"      
  comment        = "an example shared variable"
  project_ids    = [
    vercel_project.example.id
  ]
}