imp: Extract debugger-ui

[Esemesek]

Summary:

Extract debugger-ui module out of cli.

TODO:

• - Cleanup html file (extract styles, scripts and assets)
• - Improve dev scripts

Next steps:

• - Rewrite to Typescript
• - Use React

Test Plan:

• - Debugger UI working

[grabbou]

Thanks for sending this over @Esemesek and happy to see you back with PRs. Just wondering if you have applied the things we have discussed in the office already? I believe we have talked about extracting out the entire debugger middleware out to a separate package.

[Esemesek]

@grabbou I extracted the debugger UI as a middleware (https://github.com/react-native-community/cli/blob/debugger-ui/packages/cli/src/commands/server/middleware/MiddlewareManager.ts#L48). Or did you mean to extract the whole MiddlewareManager to a separate package?

EDIT: Discussed it offline, seems okay.

[grabbou]

Is this PR ready to ship / be reviewed? Also, shall we target next or master with this one? I think there's a little confusion (at least on my end) which branch is the preferred one for breaking changes from now on.

[grabbou]

nit: ../ui is just fine, path normalises for Windows by default.

[grabbou]

Let's merge it to master since it's non breaking.