Stacks UX fixes

[lionello]

• defang stack and defang stack help show help
• use AllProviders() to avoid duplication
• allow single char stack names, eg. a
• disallow hyphens in stack names (can always relax later)
• add create as alias for stack new (copilot invoked it)
• fix lint failures
• default to affordable mode for new stacks
• use existing default region logic from GetRegion()

Linked Issues

Fixes #1655

Checklist

• I have performed a self-review of my code
• I have added appropriate tests
• I have updated the Defang CLI docs and/or README to reflect my changes, if necessary