feat(config): add configuration module

Problem: need user-configurable settings with sensible defaults.

Solution: add config module that merges vim.g.todo with defaults
for data_path, default_view, default_category, date_format, and
date_syntax.

lua/todo/config.lua

@@ -0,0 +1,42 @@
+---@class todo.GcalConfig
+---@field calendar? string
+---@field credentials_path? string
+
+---@class todo.Config
+---@field data_path string
+---@field default_view 'category'|'priority'
+---@field default_category string
+---@field date_format string
+---@field date_syntax string
+---@field gcal? task.GcalConfig
+
+---@class todo.config
+local M = {}
+
+---@type todo.Config
+local defaults = {
+  data_path = vim.fn.stdpath('data') .. '/todo/tasks.json',
+  default_view = 'category',
+  default_category = 'Inbox',
+  date_format = '%b %d',
+  date_syntax = 'due',
+}
+
+---@type todo.Config?
+local _resolved = nil
+
+---@return todo.Config
+function M.get()
+  if _resolved then
+    return _resolved
+  end
+  local user = vim.g.todo or {}
+  _resolved = vim.tbl_deep_extend('force', defaults, user)
+  return _resolved
+end
+
+function M.reset()
+  _resolved = nil
+end
+
+return M