Introduces Bytes class (#1019)

This introduces a new `Bytes` standard library class, for working with
binary data.

* Add Bytes class to the standard library
* Change CLI to eval `output.bytes`
* Change code generators to map Bytes to respective underlying type
* Add subscript and concat operator support
* Add binary encoding for Bytes
* Add PCF and Plist rendering for Bytes

Co-authored-by: Kushal Pisavadia <kushi.p@gmail.com>

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

@@ -1064,3 +1064,9 @@ External reader process has already terminated.
 invalidOpaqueFileUri=\
 File URIs must have a path that starts with `/` (e.g. file:/path/to/my_module.pkl).\n\
 To resolve relative paths, remove the scheme prefix (remove "file:").
+
+invalidStringBase64=\
+`{0}` is not in base64 encoding.
+
+characterCodingException=\
+Invalid bytes for charset "{0}".