Feat: update CLI plan command's create environment message

[treysp]

The sqlmesh plan command's output includes a message about creating environments.

The message can be confusing, so this PR updates it to be consistent and clear.

For different plan scenarios, the old and new messages are:

Create prod env

• Old
  New environment prod will be created from prod
  Summary of differences against prod:
• New
  prod environment will be initialized

Create dev env without first creating prod

• Old
  New environment dev will be created from prod
  Summary of differences against dev:
• New
  dev environment will be initialized

New env with changes

• Old
  New environment dev will be created from prod
  Summary of differences against dev:
• New
  New environment dev will be created from prod
  Differences from the prod environment:

Existing env with changes

• Old
  Summary of differences against dev:
• New
  Differences from the dev environment:

Plan with no changes (existing environment)

• Old
  No differences when compared to prod
• New
  No changes to plan: project files match the prod environment

Plan with no changes (new environment)

• Old
  Error: No changes were detected. Make a change or run with --include-unmodified to create a new environment without changes.
• New
  Error: Creating a new environment requires a change, but project files match the prod environment. Make a change or use the --include-unmodified flag to create a new environment without changes.

Plan with --create-from environment that doesn't exist

• Old
  New environment dev will be created from doesnotexist
  Summary of differences against dev:
• New
  WARNING - The environment name 'doesnotexist' was passed to the plan command's --create-from argument, but 'doesnotexist' does not exist. Initializing new environment 'dev' from scratch.
  dev environment will be initialized

[georgesittas]

A couple of suggestions and a question. Feel free to ignore the suggestions if there's already alignment on the messaging.

Great work 👍