refactor(types): extract inline anonymous types into named classes (#110)

Problem: several functions used inline `{...}` table types in their
`@param` and `@return` annotations, making them hard to read and
impossible to reference from other modules.

Solution: extract each into a named `---@class`: `pending.Metadata`,
`pending.TaskFields`, `pending.CompletionItem`, `pending.SystemResult`,
and `pending.OAuthClientOpts`.

lua/pending/parse.lua

@@ -1,5 +1,12 @@
 local config = require('pending.config')
 
+---@class pending.Metadata
+---@field due? string
+---@field cat? string
+---@field rec? string
+---@field rec_mode? 'scheduled'|'completion'
+---@field priority? integer
+
 ---@class pending.parse
 local M = {}
 
@@ -515,7 +522,7 @@ end
 
 ---@param text string
 ---@return string description
----@return { due?: string, cat?: string, rec?: string, rec_mode?: 'scheduled'|'completion' } metadata
+---@return pending.Metadata metadata
 function M.body(text)
   local tokens = {}
   for token in text:gmatch('%S+') do
@@ -608,7 +615,7 @@ end
 
 ---@param text string
 ---@return string description
----@return { due?: string, cat?: string, rec?: string, rec_mode?: 'scheduled'|'completion' } metadata
+---@return pending.Metadata metadata
 function M.command_add(text)
   local cat_prefix = text:match('^(%S.-):%s')
   if cat_prefix then