feat: redirect to 404 if none router exact match

[paranoidjk]

生产环境

路由不 exact 匹配，自动跳转到 /404 (已 revert 247cc32 交由用户代码自行在项目 Layout 里面添加)

开发环境

会自动在所有嵌套子路由数组的最后添加 umi not found 页面路由，不做自动跳转

[coveralls]

Pull Request Test Coverage Report for Build 786

• 0 of 12 (0.0%) changed or added relevant lines in 2 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.04%) to 25.712%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
packages/umi-build-dev/src/DefaultLayout.js	0	6	0.0%
packages/umi-build-dev/src/plugins/404/index.js	0	6	0.0%

Files with Coverage Reduction	New Missed Lines	%
packages/umi-build-dev/src/DefaultLayout.js	1	0.0%

Totals
Change from base Build 762:	-0.04%
Covered Lines:	631
Relevant Lines:	2565

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💛 - Coveralls