Fix LSP and update plugins

- Structure LSP configuration code
- Since `lsp-zero` is deprecated, instead use `nvim-lspconfig`
- Use `nvim_lsp_signature_help` instead of `lsp_signature`

nvim/lua/config/treesitter.lua

@@ -1,6 +1,6 @@
 require'nvim-treesitter.configs'.setup {
 	-- A list of parser names, or "all" (the five listed parsers should always be installed)
-	ensure_installed = { "c", "lua", "vim", "vimdoc", "query" , "rust" },
+	ensure_installed = { "c", "lua", "vim", "vimdoc", "query" , "rust", "python" },
 
 	-- Install parsers synchronously (only applied to `ensure_installed`)
 	sync_install = false,