fix(plugin-smooth-scroll): get target element by id or name

[microJ]

Summary

Which package does this PR involve? (check one)

• [ ] root project
• [ ] vuepress-plugin-clean-urls
• [ ] vuepress-plugin-container
• [ ] vuepress-plugin-dehydrate
• [ ] vuepress-plugin-medium-zoom
• [ ] vuepress-plugin-named-chunks
• [ ] vuepress-plugin-nprogress
• [ ] vuepress-plugin-redirect
• [x] vuepress-plugin-smooth-scroll
• [ ] vuepress-plugin-table-of-contents
• [ ] vuepress-plugin-typescript
• [ ] vuepress-plugin-zooming
• [ ] vuepress-mergeable
• [ ] vuepress-types

What kind of change does this PR introduce? (check at least one)

• [x] Bugfix (fix)
• [ ] Feature (feat)
• [ ] Performance enhancement (perf)
• [ ] Code Style (style)
• [ ] Refactor (refactor)
• [ ] Docs (docs)
• [ ] Build-related changes (build)
• [ ] CI-related changes (ci)
• [ ] Testing (test)
• [ ] Other, please describe:

Does this PR introduce a breaking change? (check one)

• [ ] Yes
• [x] No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• [ ] When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

Other information:

问题描述： 复现链接，https://blog.expect2.cyou/views/front-end/2020/0824-speed-up-npm-install.html#5f447c564bdcde0006a8ffd6 ，打开控制台可以看到报错。 某些自动生成的hash以数字开头，直接使用 #123 作为 selectors 会报错。

修改内容：

1. 获取数字开头的 hash 时不报错
2. 兼容设置 name="targetAnchor" 的锚点

[meteorlxy]

兼容设置 name="targetAnchor" 的锚点

html5 中锚点不应再设置 name 了吧

[microJ]

html5 中锚点不应再设置 name 了吧

@meteorlxy

经查 w3c 标明了 name 属性是废弃的：The name attribute on the a element is obsolete. Consider putting an id attribute on the nearest container instead. 但是查证 whatwg，表示 hash-name reference that has an id or name attribute，fragment 对应元素查找过程解释在这里：https://html.spec.whatwg.org/#find-a-potential-indicated-element

这里应该是以 whatwg 为准的吧 =。=

[meteorlxy]

Thanks! ❤️