prettier: use Babylon (babel-flow) as the parser
prettier: use Babylon (babel-flow) as the parser

The `babel-flow` and `flow` parsers produce subtly different ASTs, and
Prettier's support for the latter has historically been relatively poor.

In particular, when using the `flow` parser:
  * prettier/prettier#5288: the private field sigil `#` is stripped.
    (Resolved, but not in our current version.)
  * prettier/prettier#2597: type-declaration comments are promoted to
    inline type declarations.

This commit will enable some simple uses of private fields, but not
all. See our commit 86a42582aace1b13f1e250b0a34835866b794e4a and its
associated PR (#3725) for more detail.

The more important fix (and the one that can't be obtained by updating
Prettier, at least not yet) is the second. This will allow us to rely
on Flow's typechecking in scripts which will be executed directly by
Node without a preliminary Babel run -- e.g., scripts residing in
`tools/`, or plugins for support tools such as Jest or ESLint.

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile