Get rid of string types

[LukasBombach]

To improve TS type validation and editor autocomplete support, these generic string types need to be fixed:

• https://github.com/LukasBombach/de1/blob/master/packages/de1/src/converters/index.ts#L28
• https://github.com/LukasBombach/de1/blob/master/packages/de1/src/de1.ts#L91
• https://github.com/LukasBombach/de1/blob/master/packages/de1/src/de1.ts#L137
• https://github.com/LukasBombach/de1/blob/master/packages/de1/src/de1.ts#L143-L144

[LukasBombach]

All of this is outdated. Closing.