Evaluate a provided JavaScript file or string within the initialised context.
Note that this method should only be used for initialising functions or values
within the context, no values are returned from this function. See the $call()
method for returning values.
Examples
if (FALSE) { # \dontrun{
ctx <- JSContext$new()
ctx$source(file = "path/to/file.js")
ctx$source(code = "1 + 2")
} # }