Enable different Scala versions (#340)

This updates //scala/extensions:config.bzl to provide a proper module
API that allows the user to configure different Scala versions.

For now, 2.13.12 is the latest Scala 2 version supported by rules_scala
(v6.6.0), defined in //third_party/repositories:scala_2_13.bzl. Setting
it to a later version results in an error:

```txt
File "@io_bazel_rules_scala//third_party/repositories/repositories.bzl",
    line 86, column 17, in repositories
    fail(version_message % (scala_version, repository_scala_version))

Error in fail:
Scala config (2.13.14) version does not match repository version (2.13.12)
```

Signed-off-by: Mike Bland <[email protected]>

MODULE.bazel

@@ -275,10 +275,14 @@ http_archive(
 )
 
 # This constant matches the default Scala version from rules_scala for now.
-SCALA_VERSION = "2.12.18"
+SCALA_VERSION = "2.13.12"
 SCALA_VERSIONS = [SCALA_VERSION]
 
 scala_config = use_extension("//scala/extensions:config.bzl", "scala_config")
+scala_config.settings(
+    scala_version = SCALA_VERSION,
+    scala_versions = SCALA_VERSIONS,
+)
 use_repo(
     scala_config,
     "io_bazel_rules_scala_config",