Add lsp#activate()

[mattn]

Currently, vim-lsp-settings is using FileType base event handling. The FileType event is triggered when the filetype is modified from syntax plugins. For example, *.jl is registered as filetype=lisp at $VIMRUNTIME/filetype.vim in default. But some users want to use julia with *.jl. So user install plugin. If a Language Server for common-lisp (ex: cl-lsp) is already installed, vim-lsp-settings start cl-lsp and julia-language-server both. To avoid this, vim-lsp-settings have to make little delay to check the last modified. However, vim-lsp does not have a way to activate Language Server. So this change add lsp#activate to activate Language Server in lazy.
ex:

function! s:vim_lsp_load_or_suggest_delay(ft) abort
  call timer_start(0, {timer -> [s:vim_lsp_load_or_suggest(a:ft), lsp#activate()]})
endfunction