refactor(config): split config command into subcommand packages

Split the monolithic config command into separate packages following the project's standard command structure.

Changes

• Each subcommand (get, set, edit) now in its own package
• Factory functions renamed to NewCmdGet, NewCmdSet, NewCmdEdit to match codebase patterns
• Added options struct with complete/validate/run methods
• Proper iostreams usage instead of direct cmd I/O
• Local fl variable for multiple flags
• Added test coverage for all subcommands including edit
• Tests use t.Parallel() for concurrency

Structure

internal/commands/config/
├── config.go                  (parent command)
├── get/
│   ├── config_get.go
│   └── config_get_test.go
├── set/
│   ├── config_set.go
│   └── config_set_test.go
└── edit/
    ├── config_edit.go
    └── config_edit_test.go

This refactoring brings the config command in line with the patterns used by other commands (alias, token, label) and follows all guidelines in .gitlab/duo/mr-review-instructions.yaml.

Test Results

All tests passing:

• gitlab.com/gitlab-org/cli/internal/commands/config/get ✅
• gitlab.com/gitlab-org/cli/internal/commands/config/set ✅
• gitlab.com/gitlab-org/cli/internal/commands/config/edit ✅