format code with gofumpt

gofumpt (https://github.com/mvdan/gofumpt) provides a supserset of `gofmt` / `go fmt`,
and addresses various formatting issues that linters may be checking for.

We can consider enabling the `gofumpt` linter to verify the formatting in CI, although
not every developer may have it installed, so for now this runs it once to get formatting
in shape.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

reference/regexp_test.go

@@ -525,7 +525,6 @@ func TestReferenceRegexp(t *testing.T) {
 	for i := range testcases {
 		checkRegexp(t, ReferenceRegexp, testcases[i])
 	}
-
 }
 
 func TestIdentifierRegexp(t *testing.T) {