Fix copy/paste of empty lines (#19798)

* Fix copy/paste of empty newlines again

Fixes: https://github.com/go-gitea/gitea/issues/19331
Regressed by: https://github.com/go-gitea/gitea/pull/18270

Needed to do another newline addition to the Chroma output HTML to get
copy/paste work again. The previous replacement conditions are probably
obsolete, but as I'm not 100% sure, I opted to keep them.

Specifically, the Chroma HTML change mentioned in
https://github.com/go-gitea/gitea/pull/18270#issuecomment-1013350246
broke our previous newline replacement for such empty lines.

Also included are a few changes to make the test more pleasant to work
with.

* run go mod tidy

* add util.Dedent

* copy in the code

Co-authored-by: techknowlogick <techknowlogick@gitea.io>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Lauris BH <lauris@nix.lv>

modules/util/util_test.go

@@ -225,3 +225,10 @@ func TestToTitleCase(t *testing.T) {
 	assert.Equal(t, ToTitleCase(`foo bar baz`), `Foo Bar Baz`)
 	assert.Equal(t, ToTitleCase(`FOO BAR BAZ`), `Foo Bar Baz`)
 }
+
+func TestDedent(t *testing.T) {
+	assert.Equal(t, Dedent(`
+		foo
+			bar
+	`), "foo\n\tbar")
+}