The fencing detection for blocks does not support whitespace after the tics

[ntubach]

The setup for the block detection fence regex does not allow for whitespace utilization after the tics and before the 'math' keyword.
See

markdown-katex/src/markdown_katex/extension.py

Line 31 in f868246

BLOCK_START_RE = re.compile(r"^(\s*)(`{3,}|~{3,})math")

Ex:

```math #this works
<render math>
```

``` math #this will not be detected as a markdown-katex math block
<render math>
```

I would like to update the above regex like to add in that whitespace support with something like a \s* clause
i.e. ^(\s*)(`{3,}|~{3,})\s*math