fix(syntaxes/reason-md): fewer false positives

[glebec]

Addresses, but does not fully close, issue #250.

VSCode uses Java-syntax regex instead of JS for some reason? Also, you need to escape \ characters because this is a JSON file. Finally, I am not sure what the begin is actually relevant to. However, the upshot is that using word boundary (\b) cuts down on the number of false positive hits on the ReasonML markdown code block syntax highlighting due to "re" etc. appearing anywhere in the code fence.

Maddeningly, I couldn't figure out what secret sauce would actually match on the starting triple-backtick, so stuff like this still triggers the syntax highlighting:

some re stuff
f >=> g

At some point I or someone else will have to figure out a proper regex for this. But for now this PR helps in many cases.