issue template: add syntax highlighting

We do know what languages we expect in the files
we ask the issue author to provide in the "Additional context" section,
so let's set up proper syntax highlighting for them.

.github/ISSUE_TEMPLATE/issue-template.md

@@ -26,13 +26,13 @@ Note that while these aren't strict requirements it will heavily increase your c
 -->
 
 `default.nix`/`shell.nix`/`flake.nix`:
-```
+```nix
 ```
 
 `pyproject.toml`:
-```
+```toml
 ```
 
 `poetry.lock`:
-```
+```toml
 ```