Unknown function parsing

[stefnotch]

We have to wait for the next compute-engine version https://cortexjs.io/docs/compute-engine/#(ParseLatexOptions%3Atype)

Compute-engine: parser should recognize unknown functions (like silly()) so, basically

1. check if its a known token
2. if not, check if brackets come right after it x()
3. if there are brackets, its a function
4. otherwise, split the letters and treat them as individual stuff

From Gitter

@stefnotch you can provide a parse function for xrightarrow that would parse for valid functions in the "above" argument. Arno Gourdol @arnog 12:16 For recursive functions, you have two options I can think of: use the parseUnknownToken option in the LatexSyntax parser to recognize foo as a function token. The second option would be for the definition of the := operator to create a scope (using ce.pushScope()) with a custom definition for the foo symbol. This gives you more flexibility (such as defining a domain, an argument list, etc...) in defining the symbol.