feat: replace index

[xiaohuoni]

保持 Umi@3 中的约定，index/index 会被识别成首页。

[vercel[bot]]

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1 Ignored Deployment

| Name | Status | Preview | Updated | | :--- | :----- | :------ | :------ | | **umi-next** | ⬜️ Ignored ([Inspect](https://vercel.com/chencheng/umi-next/EcRKvRF5ZrNLP1o6D4LWN64Xj4Bu)) | | Jun 21, 2022 at 9:29AM (UTC) |

[codecov[bot]]

Codecov Report

Merging #939 (1b7d90f) into master (a0cf781) will increase coverage by 0.00%. The diff coverage is 50.00%.

@@           Coverage Diff           @@
##           master     #939   +/-   ##
=======================================
  Coverage   34.15%   34.15%           
=======================================
  Files         350      350           
  Lines        8796     8797    +1     
  Branches     2050     2051    +1     
=======================================
+ Hits         3004     3005    +1     
  Misses       5455     5455           
  Partials      337      337           

Impacted Files	Coverage Δ
...ackages/preset-umi/src/features/tmpFiles/routes.ts	0.00% <0.00%> (ø)
packages/core/src/route/routesConvention.ts	97.67% <100.00%> (+0.05%)	:arrow_up:

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[xiaohuoni]

@sorrycc ci 通过了，我需要这个，提供给 antd-mobile@5 使用