fix(config): handle Windows rnvim config path

[hongyuanjia]

Summary

Fix the per-buffer rnvim_config.lua lookup so it works with Windows buffer paths. The previous slash-only Lua pattern could return nil for backslash-separated paths and fail before checking whether the config file exists.

This bug was introduced in PR #515.

Changes

• Build the config path with vim.fs.dirname() and vim.fs.joinpath() instead of a hard-coded / pattern.
• Guard the lookup when the current buffer has no file name.

[jalvesaq]

Thank you!