fix(cli): make 'koishi start' works with koishi.config.yml

[Anillc]

koishi/packages/koishi/src/worker.ts

Line 35 in d111515

if (['.yaml', '.yml'].includes(configExt)) {

使用 yml 配置文件的时候如果直接使用 koishi start 而没有指定配置文件的话这里的 configExt 会是 undefined ，修改之后如果存在文件 configExt 就不会是 undefined 从而达到加载 yml 文件的目的了

[codecov]

Codecov Report

Merging #306 (d690eb7) into develop (d111515) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #306   +/-   ##
========================================
  Coverage    92.67%   92.67%           
========================================
  Files           58       58           
  Lines        11274    11274           
  Branches      2336     2336           
========================================
  Hits         10448    10448           
  Misses         826      826           

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