feat(lsp): completion opts support custom item conversion (#30060)

Problem: Some items of completion results include function signatures that can
cause the pum to be very long when a function has many params, because pum
scales with the longest word/abbr.

Solution: add custom covert function that can customise abbr to remove params.

runtime/doc/lsp.txt

@@ -1630,6 +1630,9 @@ Lua module: vim.lsp.completion                                *lsp-completion*
     Fields: ~
       • {autotrigger}?  (`boolean`) Whether to trigger completion
                         automatically. Default: false
+      • {convert}?      (`fun(item: lsp.CompletionItem): table`) An optional
+                        function used to customize the transformation of an
+                        LSP CompletionItem to |complete-items|.
 
 
                                                  *vim.lsp.completion.enable()*

runtime/doc/news.txt

@@ -65,7 +65,9 @@ EVENTS
 
 LSP
 
-• TODO
+• Add convert field in |vim.lsp.completion.BufferOpts| of
+  |vim.lsp.completion.enable()| an optional function used to customize the
+  transformation of an Lsp CompletionItem to |complete-items|.
 
 LUA