Function: read-json / keypath (jq alternative)

[simonask]

It would be neat of there was a builtin version of this to avoid relying on having jq installed.
Something like

let werk-vscode-version = read-json "werk-vscode/package.json" | keypath ".version"

Originally posted by @jakobhellermann in #44 (comment)

[simonask]

Some design is required here: The question is whether werk should support general key-value dictionaries, or JSON operations should be special-cased.

It might be nice to also parse TOML and YAML (though the latter is annoying due to the precarious state of serde-yaml).