feat: add charsets command

[adrienaury]

No description provided.

[Copilot]

Pull Request Overview

This PR adds a new CLI command ("charsets") to list all supported charsets and updates existing commands and configuration to use the new charsets API.

• Updated charset specifications in config, fold, graph, and unfold commands to reference the new charsets package.
• Added the new "charsets" command to list available charset options.
• Introduced a charsets package implementation to centralize charset lookups and listing.

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
internal/appli/config/model_test.go	Updated charset usage in tests to use the new charsets constant.
internal/appli/config/model.go	Replaced charmap lookup loop with a call to charsets.Get for clarity.
internal/appli/command/unfold.go	Updated charset defaults to use new charsets constant.
internal/appli/command/root.go	Added a call to register the new charsets command.
internal/appli/command/graph.go	Updated charset reference in schema compilation to use charsets.
internal/appli/command/fold.go	Replaced charmap constant with charsets constant for consistency.
internal/appli/command/charsets.go	Introduced the new "charsets" command to list all supported charsets.
internal/appli/charsets/list.go	Added a package managing charset constants, listing, and lookups.

Comments suppressed due to low confidence (1)

internal/appli/command/charsets.go:51

• [nitpick] The use of the identifier 'charsets' in this file both as an imported package and as a local variable in NewCharsetsCommand may lead to confusion. Consider renaming the local variable (e.g., 'cmdInstance') to clearly distinguish it from the imported package.

for _, charset := range charsets.List() {