treesitter: make foldenable configurable

[heliosran]

Motivation

• Allow users to control whether folds are enabled by default when Treesitter folding is configured so the default behavior isn't forced by the module.

Description

• Add a new boolean option vim.treesitter.foldEnable (default false) to the Treesitter options and wire the folding setup to set vim.o.foldenable from cfg.foldEnable instead of hard-coding false.

Testing

• Attempted to parse the modified Nix files with nix-instantiate --parse modules/plugins/treesitter/treesitter.nix and nix-instantiate --parse modules/plugins/treesitter/config.nix, but nix-instantiate is not available in this environment so parsing could not be validated.

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Codex Task