Add optional "organize imports" feature

[rvanasa]

Implements "organize imports" similar to what was previously implemented in the VS Code extension. Instead of using a parsed AST, this implementation uses a token tree for more robustness to syntax errors and compatibility across a wider range of compiler versions. This implementation also confirms that all imports are preserved before applying the changes.

Example .prettierrc:

{
    "plugins": ["prettier-plugin-motoko"],
    "motokoOrganizeImports": true
}

[Kamirus]

Nice work on this feature! Solid safety-first approach (always returning the original tree on failure). I have a few suggestions

[rvanasa]

(CI error expected and fixes itself when releasing)