refactor: migrate to Neovim 0.11+ APIs

[sidlatau]

Replaces APIs deprecated in 0.11 (and slated for removal in 0.13) with their current equivalents, now that the README already requires 0.11+.

• Method-call form for client:request/notify/supports_method; drops the has("nvim-0.12") branch in on_init.
• vim.highlight → vim.hl, nvim_buf_add_highlight → nvim_buf_set_extmark. strict = false preserves the old behaviour of clamping an end column past the end of the line rather than raising.
• Devtools open through vim.ui.open, removing utils.open_command. Keeps detach = true and adds WSL support for free.
• utils.lsp_handler is gone — all three Dart handlers already use the new (err, result, ctx, config) signature.
• M.restart() uses Client:stop() + vim.lsp.start per buffer. stop() sets _is_stopping synchronously and reuse_client_default checks is_stopped(), so the first start spawns the replacement and the rest attach to it.
• Fixes includeexpr being set with vim.opt instead of vim.opt_local in the dart ftplugin, where it leaked into every other buffer.

User-visible

• New FlutterToolsDimmed highlight group, linked to 0.13's core Dimmed group where available and to Comment otherwise. It is now the default for closing tags and is used for outline return types and menu hints. Visually unchanged unless a colourscheme defines Dimmed differently.
• closing_tags.highlight now defaults to FlutterToolsDimmed rather than Comment.
• utils.open_command and utils.lsp_handler are removed. Undocumented, but reachable via require("flutter-tools.utils").