[Feature request] Git plugin: add createTime

[Mister-Hope]

Feature request

I saw V2 add a date field in frontmatter in here:

https://github.com/vuepress/vuepress-next/blob/55f780c4a516b2c0119328874906647af80ac73b/packages/%40vuepress/plugin-git/src/types.ts#L5-L15

And add a date field in page:

https://github.com/vuepress/vuepress-next/blob/55f780c4a516b2c0119328874906647af80ac73b/packages/%40vuepress/core/src/types/page.ts#L100-L105

Theme and plugin builders may want to fetch the page date and use it.
The date can also be infered from git log, but git plugin currently support only updatedTime and contributors.

https://github.com/vuepress/vuepress-next/blob/55f780c4a516b2c0119328874906647af80ac73b/packages/%40vuepress/plugin-git/src/types.ts#L5-L15

I think adding a new field called createTime(first commit) would be better for theme and plugin builders to fetch the page date from git as a improvement instead of letting users fill in.

[meteorlxy]

I also considered to add that, but I'm not sure if it's really useful:

1. The time of first commit might not be the publish time of an article.
2. Blog users may prefer to put date in filename / directory name ? (Not sure about that)

[meteorlxy]

But as a plugin, providing this feature makes sense