Feature: Support Custom Environments in Deployment

[lolJS]

Feature Request: Support Custom Environments via target Parameter in vercel_deployment

Description:
Currently, the vercel_deployment resource within the terraform-vercel-provider supports predefined deployment targets (production, preview, and development). However, it does not support deployments to custom environments defined in Vercel (e.g., "staging" or other named environments created by users).

Adding support for custom environment targets would greatly enhance the flexibility and usability of the terraform-vercel-provider, enabling deployments to user-defined environments directly through Terraform.

Proposed Solution:
Extend the target parameter in the vercel_deployment resource to accept custom environment names as strings. For example:

resource "vercel_deployment" "example" {
  project_id = "your_project_id"
  target     = "staging" # custom environment name
  files      = [
    // files
  ]
}