Seperate type and normal imports

[JeremyMoeglich]

This commit separates imports like

import { Message, MessageAttachment, MessageHeaders } from './message';

to

import type { MessageAttachment, MessageHeaders } from './message';
import { Message } from './message';

this is needed to not throw an Error when using the typescript compiler options:
"preserveValueImports",
"isolatedModules"

[zackschuster]

i'd toyed with this in the past, but it raised issues downstream. the main rub is that this raises the minimum required version of typescript to 3.8.

@eleith it's been two years, so perhaps we're safe in using this feature now without it being breaking? or we can take the chance to bump to 4.0 with other changes (min. node 12, etc)?

[eleith]

i'm not sure how to evaluate typescript version adoption. with that said, personally, i'ld be up for a major version change to increase our min node and typescript versions.

[zackschuster]

i agree. i'll merge this and work on the remaining tasks for closing #292.