Defer global dictionary parsing until first lookup

[Copilot]

Global SKK dictionaries were parsed eagerly during initialization, increasing startup cost even when unused. This change defers file loading and parsing until the dictionary is queried.

• Lazy loading
  • Add file-backed dictionary construction via Dictionary.fromFile().
  • Load and parse on the first conversion or completion request.
  • Share concurrent load operations through a single promise.
• Source initialization
  • Return lazy dictionaries from the SKK dictionary source instead of parsing immediately.
• Regression coverage
  • Verify that file access is deferred until lookup.

const dictionary = Dictionary.fromFile(path, encoding);
// File is loaded only when queried.
await dictionary.getHenkanResult("okurinasi", "かな");

[Shougo]

@copilot

Task lint deno lint denops
error[require-await]: Async method 'getDictionaries' has no 'await' expression or 'await using' declaration.
Error:   --> /home/runner/work/skkeleton/skkeleton/repo/denops/skkeleton/sources/skk_dictionary.ts:26:3
   | 
26 |   async getDictionaries(): Promise<BaseDictionary[]> {
   |   ^^^^^
   = hint: Remove 'async' keyword from the function or use 'await' expression or 'await using' declaration inside.

  docs: https://docs.deno.com/lint/rules/require-await


error[require-await]: Async method 'getDictionaries' has no 'await' expression or 'await using' declaration.
Error:   --> /home/runner/work/skkeleton/skkeleton/repo/denops/skkeleton/sources/skk_dictionary.ts:26:3
   | 
26 |   async getDictionaries(): Promise<BaseDictionary[]> {
   |   ^^^^^
   = hint: Remove 'async' keyword from the function or use 'await' expression or 'await using' declaration inside.

  docs: https://docs.deno.com/lint/rules/require-await


error[require-await]: Async method 'getDictionaries' has no 'await' expression or 'await using' declaration.
Error:   --> /home/runner/work/skkeleton/skkeleton/repo/denops/skkeleton/sources/skk_dictionary.ts:26:3
   | 
26 |   async getDictionaries(): Promise<BaseDictionary[]> {
   |   ^^^^^
   = hint: Remove 'async' keyword from the function or use 'await' expression or 'await using' declaration inside.

  docs: https://docs.deno.com/lint/rules/require-await


Found 3 problems
``` 直してください

[Copilot]

@copilot

...

getDictionaries から async を削除し、Promise.resolve(...) を返すよう修正しました。Deno は環境にないため lint は実行できませんでしたが、変更はコミット済みです（f2a1d6e）。