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/highlight/highlight.go

@@ -203,6 +203,8 @@ func File(numLines int, fileName, language string, code []byte) []string {
 			content = "\n"
 		} else if content == `</span><span class="w">` {
 			content += "\n</span>"
+		} else if content == `</span></span><span class="line"><span class="cl">` {
+			content += "\n"
 		}
 		content = strings.TrimSuffix(content, `<span class="w">`)
 		content = strings.TrimPrefix(content, `</span>`)