feat: respect cindent and smartindent

[sharpchen]

Vim/Neovim has three strategies of indentation, smartindent, cindent, indentexpr, see :h C-indenting. Current implementation does not respect the other two options when indentexpr is empty. So this pr added support for cindent and smartindent, and with some other changes for better adaption of indentexpr.

• use vim.api.nvim_eval for safe_eval so that it can evaluate indentexpr with parameter within expr such as GetCSIndent(v:lnum)
• lang is not always filetype, fixed with a non-exhaustive manual switch