Implement SPICE-0009 External Readers (#660)

This adds a new feature, which allows Pkl to read resources and modules from external processes.

Follows the design laid out in SPICE-0009.

Also, this moves most of the messaging API into pkl-core

pkl-core/src/main/resources/org/pkl/core/errorMessages.properties

@@ -1081,3 +1081,33 @@ Malformed proxy URI (expecting `http://<host>[:<port>]`): `{0}`.
 
 cannotAnalyzeBecauseSyntaxError=\
 Found a syntax error when parsing module `{0}`.
+
+malformedMessageHeaderLength=\
+Malformed message header (expected size 2, but got {0}).
+
+malformedMessageHeaderException=\
+Malformed message header.
+
+malformedMessageHeaderUnrecognizedCode=\
+Malformed message header (unrecognized code `{0}`).
+
+unhandledMessageCode=\
+Unhandled decoding message code `{0}`.
+
+unhandledMessageType=\
+Unhandled encoding message type `{0}`.
+
+malformedMessageBody=\
+Malformed message body for message with code `{0}`.
+
+missingMessageParameter=\
+Missing message parameter `{0}`
+
+unknownRequestId=\
+Received response {0} for unknown request ID `{1}`.
+
+externalReaderFailure=\
+Failed to communicate with external reader process.
+
+externalReaderDoesNotSupportScheme=\
+External {0} reader does not support scheme `{1}`.