This prevents a selection from inheriting the default gray background when editor is defocused.

To make closebrackets work with this mode

Closes #4277

Closes #4276

Ensure that when an overlay is added that the class `cm-overlay` is
used to denote a `span` instead of `cm-cm-overlay` (with the `cm-`
prefix repeated twice).

* E.g. before fix:
```
<span class="cm-cm-overlay cm-spell-error">zpelling</span>
```

* And after fix:
```
<span class="cm-overlay cm-spell-error">zpelling</span>
```

Issue #4261

- remove non-swifty keywords
- remove repeated keywords
- rearrange in more logical way

Closes #4258

The kludge of pulling files from github no longer works now
that we have a build process, and since most people have their
own build pipeline now, I don't feel that supporting on-website
builds is that important anymore.

Closes #4255

Closes #4254

Closes #4253

This reverts commit 8a1dd160.

Issue #4123

Indentation should happen top-to-bottom so that subsequent
indents are based on the updated code above

Issue #4248

Closes #4248

Closes #4233

Closes #4214