feat: adding --start-cmd flag to heroku local when no Procfile is present

[michaelmalave]

Summary

This adds a --start-cmd flag so a user can specify a command to run when there is no Procfile present.

Type of Change

Breaking Changes (major semver update)

• Add a ! after your change type to denote a change that breaks current behavior

Feature Additions (minor semver update)

• feat: Introduces a new feature to the codebase

Patch Updates (patch semver update)

• fix: Bug fix
• deps: Dependency upgrade
• revert: Revert a previous commit
• chore: Change that does not affect production code
• refactor: Refactoring existing code without changing behavior
• test: Add/update/remove tests

Testing

Go to a project that leverages a Procfile on your local machine.

Notes:

Steps:
run with Procfile

/path/to/cli/bin/run local

run with Procfile and command

/path/to/cli/bin/run local --start-cmd 'echo 1'  

run with no Procfile and command

mv Procfile zProcfile
/path/to/cli/bin/run local --start-cmd 'echo 1'  

run with no Procfile and no command

mv Procfile zProcfile
/path/to/cli/bin/run local   

rename your Procfile

mv zProcfile Procfile

Screenshots (if applicable)

Related Issues

GitHub issue: #3630
GUS work item: [WI number](WI link)

[eablack]

this looks great! couple clarifying questions