Use MessageContent instead of string in TS typings

[retrixe]

Since Eris' CommandClient supports the CommandGenerator returning an Object, this commit edits the TypeScript typings to support a function returning one.

[retrixe]

eris/lib/command/Command.js

Line 12 in 4e229d5

* @arg {Function | String | Array<Function | String>} generator A response string, array of functions or strings, or function that generates a string or array of strings when called.

eris/lib/command/CommandClient.js

Line 303 in 4e229d5

* @arg {Function | String | Array<Function | String>} generator A response string, array of functions or strings, or function that generates a string or array of strings when called.

Documentation changes required here?

[abalabahaha]

The JSDoc generator has a couple issues with custom types, seems fine as is