I just got bitten by this while preparing this PR:

    liferay/liferay-js-themes-toolkit#247

Basically, given an object containing some version numbers properties:

```
{
  '6.2': true,
  '7.0': true,
  '7.1': true,
}
```

we currently have ESLint configured via "quote-props" to require quotes
waround property names "as-needed"; producing this:

```
{
  6.2: true,
  '7.0': true,
  7.1: true,
}
```

This looks pretty terrible. I noticed that Prettier didn't have an
opinion on this, so I changed them all to skip the quotes:

```
{
  6.2: true,
  7.0: true,
  7.1: true,
}
```

of course, that is wrong too, because `7.0` actually gets stringified to
`'7'`, meaning the object is equivalent to:

```
{
  '6.2': true,
  '7': true,
  '7.1': true,
}
```

This breaks checks of the form `if (object['7.0'] === true)`. So I added
a lint suppression and put strings back on all the keys, but we should
just configure ESLint to use "consistent-as-needed" instead. The
docs:

    https://eslint.org/docs/rules/quote-props):

describe it like so:

> requires quotes around all object literal property names if any name
> strictly requires quotes, otherwise disallows quotes around object
> property names

Test plan: tweak my local copy of eslint-config-liferay in the repo with
the PR I linked at the top and confirm that all-quotes are enforced in
that object but not in other places. It's not a total bed of roses,
unfortunately. I found that if you start with an object like this:

```
{
  a: 1,
  'b-c': 2,
}
```

it will get changed to:

```
{
  "a": 1,
  'b-c': 2,
}
```

on the bright side, if you then change it to use single quotes, it stays
that way. Closest ticket I can find for an issue like this is:

microsoft/vscode-eslint#209

So, it's a wrinkle, but I still think we're better off with this change
than without it.

Closes: https://github.com/liferay/eslint-config-liferay/issues/38

As noted in this comment:

#39 (comment)

we need the latest Prettier (1.17.0) in order to make use of the
"consistent-as-needed" rule, and in our Prettier config, we'll need to
use `"quote-props": "consistent"`.

So, I updated everything using `yarn upgrade-interactive` and added the
necessary rule to the ".prettierrc.json" file; other projects will need
to do the same.