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/complete.lua

@@ -1,5 +1,9 @@
 local config = require('pending.config')
 
+---@class pending.CompletionItem
+---@field word string
+---@field info string
+
 ---@class pending.complete
 local M = {}
 
@@ -32,7 +36,7 @@ local function get_categories()
   return result
 end
 
----@return { word: string, info: string }[]
+---@return pending.CompletionItem[]
 local function date_completions()
   return {
     { word = 'today', info = "Today's date" },
@@ -89,7 +93,7 @@ local recur_descriptions = {
   ['2y'] = 'Every 2 years',
 }
 
----@return { word: string, info: string }[]
+---@return pending.CompletionItem[]
 local function recur_completions()
   local recur = require('pending.recur')
   local list = recur.shorthand_list()