fix(pack): use before_init instead of on_new_config for schemastore

Closes #1774

vim.lsp.config does not support on_new_config (neovim/neovim#32287).
Switch to before_init with nil guards, matching the pattern used by
basedpyright and vue packs.

Author: syltharion

lua/astrocommunity/pack/json/init.lua

@@ -11,7 +11,9 @@ return {
           ---@diagnostic disable: missing-fields
           config = {
             jsonls = {
-              on_new_config = function(config)
+              before_init = function(_, config)
+                if not config.settings then config.settings = {} end
+                if not config.settings.json then config.settings.json = {} end
                 if not config.settings.json.schemas then config.settings.json.schemas = {} end
                 vim.list_extend(config.settings.json.schemas, require("schemastore").json.schemas())
               end,

lua/astrocommunity/pack/yaml/init.lua

@@ -11,7 +11,9 @@ return {
           ---@diagnostic disable: missing-fields
           config = {
             yamlls = {
-              on_new_config = function(config)
+              before_init = function(_, config)
+                if not config.settings then config.settings = {} end
+                if not config.settings.yaml then config.settings.yaml = {} end
                 config.settings.yaml.schemas = vim.tbl_deep_extend(
                   "force",
                   config.settings.yaml.schemas or {},