Syntax revision for markdown fenced code attributes

[IllustratedMan-code]

Describe your proposed improvement and the problem it solves. The current method of defining fenced code attributes breaks syntax highlighting in git hosting sites (tested in github, codeberg).

For example:

```{.python tangle="output.py"}
print("hi there")
```

renders as

print("hi there")

I propose a minor change to the syntax that is already partially supported:

```python {tangle="output.py"}
print("hi there")
```

Which renders as:

print("hi there")

Code blocks of the form:

```python
print("hi there")
```

already work in pandoc, but additional attributes are not parsed as attributes, they are parsed as part of the codeblock text.

Describe alternatives you've considered. The pandoc-entangled repository works around this problem by specifying a custom link before code blocks. This is definitely not ideal, because it breaks write format interchangeability within pandoc.