URI standard library

[bioball]

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.

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.

[bioball]

SPICE is up! apple/pkl-evolution#13