support default and custom mimes for `serveStatic`

[pi0]

Discussed in https://github.com/orgs/h3js/discussions/1135

H3 built-in serveStatic utility can support mimes for common file extensions to avoid sending them as application/octet-stream.

We cannot depend on external packages to keep package size minimal but a list of common extensions can be aded.

We probably also need to support a new option to allow custom mime resolution with more extensions.

[gulshan]

Ran into this issue in v2 and got around it by-

getContents: (id) => readFile(path.join(staticDir, id)).then(buf => new Blob([buf])),