fix[devtools/standalone]: update webpack configurations (#26963)

## Summary
Overlooked when was working on
https://github.com/facebook/react/pull/26887.

- Added `webpack` packages as dev dependencies to `react-devtools-core`,
because it calls webpack to build
- Added `process` package as dev dependency, because it is injected with
`ProvidePlugin` (otherwise fails with Safari usage)
- Updated rule for sourcemaps
- Listed required externals for `standalone` build

Tested on RN application & Safari