feat: allow the user to specify the frontmatter property names

[Sparticuz]

Hello, new to the project here, so please forgive if there is an existing contributing workflow I'm unaware of.

My frontmatter has different property names than lastmod.ts expected, so I updated the options and parsing code to allow for custom property names.

Originally, I had the three properties as string instead of string[], however, that might have been a breaking change. Specifically, the modified property had three options available, which necessitated needing string[], which keeps existing functionality

Plugin.CreatedModifiedDate({
  priority: ["frontmatter"],
  properties: {
    created: ["created"],
    modified: ["updated"],
    published: ["publishDate"],
  },
}),

Thanks!