feat: use config from _G.packer_plugins in packer_compiled.lua

[younger-1]

Since plugins's config is in config field of _G.packer_plugins, we could use it directly if the type of config field is fnction. Otherwise, the compiled config string could be appear twice in packer_compiled.lua.

Result:

-- Config for: monokai.nvim
time([[Config for monokai.nvim]], true)
try_loadstring(_G.packer_plugins["monokai.nvim"].config[1], "config", "monokai.nvim")
time([[Config for monokai.nvim]], false)

This doesn't impact if the type of config field is string, so it works as expect.