Flow: enable exact_by_default (#25220)

With this change, a simple object type `{ }` means an exact object `{| |}` which most people assume.
Opting for inexact requires the extra `{ a: number, ... }` syntax at the end.

A followup, someone could replace all the `{| |}` with `{ }`.

scripts/flow/config/flowconfig

@@ -37,18 +37,17 @@
 
 [lints]
 untyped-type-import=error
-implicit-inexact-object=error
 
 [options]
 server.max_workers=4
 esproposal.class_static_fields=enable
 esproposal.class_instance_fields=enable
 esproposal.optional_chaining=enable
+exact_by_default=true
+munge_underscores=false
 
 # Substituted by createFlowConfig.js:
 %REACT_RENDERER_FLOW_OPTIONS%
 
-munge_underscores=false
-
 [version]
 ^0.122.0