Problem:
Two cases lsp.enable() won't work in the first FileType event
1. lsp.enable luals inside FileType or ftplugin/lua.lua, then:
```
nvim a.lua
```
2. lsp.enable luals inside FileType or ftplugin/lua.lua, then:
```
nvim -> :edit a.lua -> :mksession! | restart +qa! so Session.vim
```
Solution:
Currently `v:vim_did_enter` is used to detected two cases:
1. "maunally enabled" (lsp.enable() or `:lsp enable`)
2. "inside FileType event"
To detect 2. correctly we use did_filetype().
fd45bc8cab