URI standard library

Pkl's standard library is missing a URI type, but this is a quite common type in service configuration.

For example:

* Website address
* Database connection string

Additionally, it's necessary to construct a valid URI sometimes when interacting with `read()`.
Currently, this requires [importing package `pkl.experimental.uri`](https://github.com/apple/pkl-pantry/blob/main/packages/k8s.contrib/convert.pkl#L112).

There should be an API within the standard library for working with URIs.

This requires a SPICE to be written to iterate on the design.

Start by reviewing the existing experimental URI package and its use in packages/k8s.contrib/convert.pkl around line 112. The issue calls for a SPICE to iterate on the standard-library URI API design; done means the design is agreed and the standard-library work is scoped.

Written by the indexing model from the issue text.