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/lsp/mason.lua

@@ -0,0 +1,4 @@
+require('mason').setup({})
+require('mason-lspconfig').setup({
+	ensure_installed = {'rust_analyzer', 'pylsp'},
+})