enhancement: move thor & disco to runtime package

[darrenvechain]

Description

Create a runtime package inside ./cmd/thor to handle starting nodes. The original main.go now just proxies directly into runtime with os.Args

This allows us to:

• easily start a thor instance for integration testing
• testing runtime start flag combinations

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [x] New and existing E2E tests pass locally with my changes
• [x] Any dependent changes have been merged and published in downstream modules
• [x] I have not added any vulnerable dependencies to my code