Step Functions: support number coercision with Number(), +<expr> and -<expr>

[sam-goodwin]

• + <expr> should coercse ` to a number, see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Unary_plus
• - <expr> should coercse ` to a number, see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Unary_minus
• Number(<expr>) should coercse <expr. to a number

[thantos]

Sfn supports to string... But I don't think it'll support from string to number...

As far as I know at least, unless there is some Json path magic.