Move global variable declarations after environment variable declarations

[DannyBen]

This PR moves the global variables declarations after the environment_variables declarations.

This is designed to allow populating global variables with values from environment variables, after they were handled by the bashly initialization process.

For example:

# bashly.yml
environment_variables:
- name: cli_config_file
  help: Path to config file
  default: config.yaml

variables:
- name: config_file
  value: $CLI_CONFIG_FILE

will now work, and generate this snippet in initialize():

export CLI_CONFIG_FILE="${CLI_CONFIG_FILE:-config.yaml}"
declare -g config_file="$CLI_CONFIG_FILE"

From this point on, this behavior is guaranteed using a fixture test.