feat(lsp): use fuzzy match on filterText instead of prefix match

The `complete()` mechanism matches completion candidates against
the typed text, so strict pre-filtering isn't necessary.

This is a first step towards supporting postfix snippets (like
`items@insert` in luals)

test/functional/plugin/lsp/completion_spec.lua

@@ -228,7 +228,7 @@ describe('vim.lsp.completion: item conversion', function()
           },
         },
         {
-          filterText = 'notthis_thread',
+          filterText = 'no_match',
           insertText = 'notthis_thread',
           insertTextFormat = 1,
           kind = 9,