Un-deprecate Resource.base64, compute bytes/base64 in terms of each other (#1138)

This makes changes to avoid a needless breaking change.
It preserves code like `new Resource { base64 = someValue }.sha256`,
and also code that renders `Resource` into static formats.

Co-authored-by: Islon Scherer <islonscherer@gmail.com>

pkl-server/src/test/kotlin/org/pkl/server/AbstractServerTest.kt

@@ -632,7 +632,7 @@ abstract class AbstractServerTest {
             content
             
             $tripleQuote
-          bytes = Bytes(99, 111, 110, 116, 101, 110, 116, 10)
+          base64 = "Y29udGVudAo="
         }
         res2 {
           uri = "modulepath:/dir1/resource1.txt"
@@ -640,7 +640,7 @@ abstract class AbstractServerTest {
             content
             
             $tripleQuote
-          bytes = Bytes(99, 111, 110, 116, 101, 110, 116, 10)
+          base64 = "Y29udGVudAo="
         }
         res3 {
           ressy = "the module2 output"