[DX] standard formatter

[xieyuheng]

Problem:

We need a standard formatter.

To help users avoid arguing about code formating.

Solution 1:

Implement formatExp and formatStmt(s).

These functions should take formatting context as argument,
and recurse down the Exp and Stmt.

Solution 2:

Learn from prettier API, and write prettier plugin.

https://prettier.io/docs/en/plugins.html

[xieyuheng]

Will be useful in online playground: https://cicada-lang.org/playground/