Add support for environment variable validations

[DannyBen]

Adds support for using validate (custom validations) in environment variables.

Discovered in #545

Todo

• Initial pilot
• Confirmed implementation
• Example
• Tests
• Update bashly YAML/JSON schema
• Update strings JSON schema
• Update bashly docs command
• Specs pass
• Rubocop pass
• Update doc site

Sample bashly.yml

name: cli

environment_variables:
- name: release_dir
  help: Path to the releases folder
  default: release
  validate: dir_exists
- name: log_dir
  help: Directory for storing log files
  default: "${RELEASE_DIR}/logs"
  validate: dir_exists

[DannyBen]

@EmilyGraceSeville7cf - would you mind double checking my schema changes?

[EmilyGraceSeville7cf]

LGTM 😄 I have some notes about changes, but they are not required to be applied.

[EmilyGraceSeville7cf]

Suggested change

	"type": "string",
	"type": "string",
	"minLength": 1,
	"pattern": "^\\s+|\\s+$"

I suggest adding more validations. I am not sure whether spacing matters, but I would prefer to have spacing checks done.

[DannyBen]

Thank you. I applied the correction in the strings.json. As for the bashly.json, I am leaving it as is. Rationale:

1. the bashly.json is generated from support/schema/bashly.yml
2. the new validate in environment_variables is a copy-paste from the same function in args and flags
3. I prefer these schemas to be a guide rather than being a complete lockdown on the exact requirements. Easier to maintain.

If you feel strongly otherwise, I might accept a separate PR with changes to bashly.yml rather than the JSON artifact.