fix(lsp): unify get_completion_word for textEdits/insertText

Problem:

After https://github.com/neovim/neovim/pull/32377 selecting snippets
provided by luals inserted the multi-line text before accepting the
candidates. That's inconsistent with servers who provide `textEdit`
instead of `insertText` and having lines shift up/down while cycling
through the completion candidates is a bit irritating.

Solution:

Use the logic used for `textEdit` snippets also for `insertText`

test/functional/plugin/lsp/completion_spec.lua

@@ -562,12 +562,23 @@ describe('vim.lsp.completion: item conversion', function()
           range = range0,
         },
       },
+      -- luals for snippet
+      {
+        insertText = 'for ${1:index}, ${2:value} in ipairs(${3:t}) do\n\t$0\nend',
+        insertTextFormat = 2,
+        kind = 15,
+        label = 'for .. ipairs',
+      },
     }
     local expected = {
       {
         abbr = 'copyOf(Collection<? extends E> coll) : List<E>',
         word = 'copyOf',
       },
+      {
+        abbr = 'for .. ipairs',
+        word = 'for .. ipairs',
+      },
       {
         abbr = 'insert',
         word = 'insert',