Add a prefix to Flow specific AST node type

[fisker]

Add a prefix to the type of Node.

1. Avoid future conflicts with new ES syntax. Eg: we already support Match syntax, but there is a proposal ECMAScript Pattern Matching, once it reachs higher stage, most likly ESTree will use Pattern or Match as a prefix for these nodes.
2. Prettier currently supports 330 types of node (exclude 8 from flow v0.294), sometimes it's hard to tell which node is from Flow, typescript-eslint prefixes the node with TS, and our Angular related node also use NG as a prefix, they are easy to identify.

[gkz]

Something for us to consider. If we are going to redo our AST node output, now that Flow has adopted most of TypeScript's syntax, should we output TS-prefixed nodes where the syntax is the same? That would simplify some tooling. There would still be differences that would need to be handled however.

[fisker]

I'm not sure if this is a good idea.

TSFoo with a parent FLOWBar seems weird to me, and may also confuse.

If we want to align with TypeScript, I assume we will follow typescript-eslint.

There are some bad designs for some of their AST shape. For example, the current AST in Flow for as const makes more sense to me. I already asked them to update the AST in the past few years, will we strictly follow?

On the other hand,

That would simplify some tooling.

This is true.

So, I'm fine with whatever decision you make.