feat(config): add recur_syntax and someday_date fields

Problem: the plugin needs configuration for the recurrence token name
and the sentinel date used by the `later`/`someday` named dates.

Solution: add `recur_syntax` (default 'rec') and `someday_date`
(default '9999-12-30') to pending.Config and the defaults table.

lua/pending/config.lua

@@ -8,6 +8,8 @@
 ---@field default_category string
 ---@field date_format string
 ---@field date_syntax string
+---@field recur_syntax string
+---@field someday_date string
 ---@field category_order? string[]
 ---@field drawer_height? integer
 ---@field gcal? pending.GcalConfig
@@ -22,6 +24,8 @@ local defaults = {
   default_category = 'Todo',
   date_format = '%b %d',
   date_syntax = 'due',
+  recur_syntax = 'rec',
+  someday_date = '9999-12-30',
   category_order = {},
 }