Add spellchecker (electron-spellchecker)

[adlk]

This is an alternative approach to #753 that is using electron-spellchecker.

I'm extending some electron-spellchecker classes to provide more flexibility and to mimic the setup of #753

This PR introduces the new make targets:

• make dev: starting the webpack dev server
• make start: start electron and check if a native dependency rebuild is necessary
• make rebuild-deps: rebuilds native dependencies. This is a dependency of make start

[adlk]

@mirka

Can we keep all the file/folder names spinal-cased to match the rest of the codebase?

yup, I will update the file/folder names.

Also, I initially had some trouble getting this to work in my environment. Turns out it was a NODE_MODULE_VERSION issue like the one described here, but it was hard to notice there was even such an error because it was only thrown in the devtools console.

I'm afraid electron-spellchecker will break rather silently like this whenever it's run in a node version it doesn't expect. (Is that a valid concern?) Should we be adding an electron-rebuild step somewhere?

I'm planning to add the same approach using electron-rebuild as we're doing in wp-desktop to check if a rebuild is necessary.
https://github.com/Automattic/wp-desktop/blob/develop/Makefile#L147-L149

Also, I'm planning a PR that is simplifying debugging when we are in dev mode like introducing keyboard shortcuts for showing dev tools, ...

[adlk]

@mirka @roundhill the required changes were made. Plus,

1. I've added the make targets:

   • make dev: starting the webpack dev server
   • make start: start electron and check if a native dependency rebuild is necessary
   • make rebuild-deps: rebuilds native dependencies. This is a dependency of make start

2. Spellchecker is now just used in fields where we really want spell checking to happen.

I'm going to test everything once more and then we should be done here.

[mirka]

Worked great 👍

[mirka]

The TagList is showing a context menu when it shouldn't, but it was a markup problem (#844) and not a problem with this spellchecker PR.

Works great, thank you!