eslint: Add missing space-before-keywords rule

[adrienverge]

All examples in README.md seem to agree on avoiding missing whitespaces before keywords such as:

if (cond) {
}else {       // no space before 'else'
}

try {
}catch (e) {  // no space before 'catch'
}

This patchs adds the space-before-keywords rule as an error (as is space-after-keywords already).

[ljharb]

LGTM 👍

(altho it might be nice to specify [2, "always"] on both keyword spacing rules to make it clear what behavior we want, even though it's the default)

[adrienverge]

@ljharb Good idea, done! Also added "always" to space-after-keywords to be clear.

[ljharb]

Thanks, this looks great! I'll defer merging this to @justjake.

[justjake]

👍

[taion]

Isn't the 'always' redundant here, given that it's the default?

[ljharb]

s/redundant/explicit. Just because it's the default doesn't mean we shouldn't explicitly specify it.