refactor(lsp)!: rename lsp.completion.trigger() to get() (#32911)

Problem: `trigger` is a custom word not yet used in APIs. 

Solution: Use `get` instead because the main effect is that the 
completion candidates will be collected (and shown by default,
but follow-up commits are planned to add an `on_result` callback
that allows more general handling).

---------

Co-authored-by: Christian Clason <c.clason@uni-graz.at>

test/functional/plugin/lsp/completion_spec.lua

@@ -847,7 +847,7 @@ describe('vim.lsp.completion: protocol', function()
     exec_lua(function()
       local win = vim.api.nvim_get_current_win()
       vim.api.nvim_win_set_cursor(win, pos)
-      vim.lsp.completion.trigger()
+      vim.lsp.completion.get()
     end)
 
     retry(nil, nil, function()
@@ -1153,7 +1153,7 @@ describe('vim.lsp.completion: protocol', function()
         end,
       })
 
-      vim.lsp.completion.trigger()
+      vim.lsp.completion.get()
 
       return params
     end)