Fix FIDL library parsing to allow deprecated_syntax

As of Ifb06be9cd1eaab780f59f6398095b05ee1e75958, all FIDL files in tree
have `deprecated_syntax;` at the top, used for the transition to the new
syntax. This broke fidlbolt's regex parsing, so importing SDK libraries
stopped working. This CL fixes that.

Bug: 70247
Test: manually tried `deprecated_syntax; library fuchsia.io;`
Change-Id: I21dcabf90e3cbaf3b5b829919c7d42e5cddcd134
Reviewed-on: https://fuchsia-review.googlesource.com/c/fidlbolt/+/545885
Reviewed-by: Felix Zhu <fcz@google.com>
diff --git a/backend/analyze.go b/backend/analyze.go
index 38f8ef5..3a50833 100644
--- a/backend/analyze.go
+++ b/backend/analyze.go
@@ -385,6 +385,9 @@
 	// the file (only comments and whitespace can precede it).
 	libraryRegexp = regexp.MustCompile(`` +
 		`^(?:\s*//[^\n]*\n)*\s*` +
+		// TODO(fxbug.dev/70247): Remove the next two lines.
+		`(?:deprecated_syntax\s*;)?` +
+		`(?:\s*//[^\n]*\n)*\s*` +
 		`library\s+(` +
 		fidlIdentifierPattern +
 		`(?:\.` + fidlIdentifierPattern + `)*` +