syntax: Add `json_encode` to stdlib

[braunsonm]

Request

Add a function to the standard library similar to json_decode but in the reverse direction. Taking a map type and encoding to a string.

Use case

In order to configure the embedded blackbox, it would be nice to be able to use the powerful features of Alloy syntax while providing the inline configuration.

prometheus.exporter.blackbox "example" {
  config = json_encode({
    modules = {
    ...
    }
  })

  target "example" {
    address = "http://example.com"
    module  = "http_2xx"
  }

  target "grafana" {
    address = "http://grafana.com"
    module  = "http_2xx"
  }
}

[rfratto]

Interesting idea, I wouldn't be against having a json_encode function :)

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