feat(defaults): enable 'termguicolors' by default when supported by terminal

Enable 'termguicolors' automatically when Nvim can detect that truecolor
is supported by the host terminal.

If $COLORTERM is set to "truecolor" or "24bit", or the terminal's
terminfo entry contains capabilities for Tc, RGB, or setrgbf and
setrgbb, then we assume that the terminal supports truecolor. Otherwise,
the terminal is queried (using both XTGETTCAP and SGR + DECRQSS). If the
terminal's response to these queries (if any) indicates that it supports
truecolor, then 'termguicolors' is enabled.

runtime/lua/vim/_meta.lua

@@ -20,6 +20,7 @@ vim.lsp = require('vim.lsp')
 vim.re = require('vim.re')
 vim.secure = require('vim.secure')
 vim.snippet = require('vim.snippet')
+vim.text = require('vim.text')
 vim.treesitter = require('vim.treesitter')
 vim.ui = require('vim.ui')
 vim.version = require('vim.version')