Add missing conversion method on String, fixes #7

The message was already understood by FSDirectoryEntry,
FSLocator, and FSReference.

In general, asFileReference is probably even the better selector
than asReference because the latter is so unspecific about what
kind of reference it means.

Author: j4yk

src/FS-Core.package/String.extension/instance/asFileReference.st

@@ -0,0 +1,4 @@
+*fs-core-converting
+asFileReference
+	"Return an FSReference on disk"
+	^ FileSystem disk referenceTo: self

src/FS-Core.package/String.extension/methodProperties.json

@@ -4,6 +4,7 @@
 	"instance" : {
 		"asDirectory" : "CamilloBruni 1/19/2012 14:20",
 		"asFile" : "CamilloBruni 1/19/2012 14:20",
+		"asFileReference" : "jr 11/8/2021 21:18",
 		"asPath" : "jr 3/2/2017 17:39",
 		"asPathWith:" : "cwp 11/21/2009 11:30",
 		"asReference" : "jr 3/2/2017 17:39",