feat: add --output flag and deprecate --format for variable export
Description
- feat: add --output flag and deprecate --format for variable export
Adds a new --output flag as the primary flag for specifying output
format in glab variable export, aligning with other commands like
glab mr list and glab token list which use --output.
The existing --format flag is kept for backward compatibility but marked as deprecated. Both flags work identically by pointing to the same variable.
Related Issues
Resolves #[issue_number]