document help function in jlox REPL

[zxul767]

there is now a handy function help which can be passed any value and it will print whatever help is available. for example:

>>> help(sin)
{ sin(n:number) -> number } : <built-in function>

>>> help("hello".starts_with)
{ starts_with(prefix:str) -> bool } : <built-in method>

we should advertise it in the initial help banner of the REPL as well as in the READMEs