Implement cursor indexing through chat input

[jmshgs]

Description

This implements cursor indexing through the user's chat input. Fixes #160.

Type of change

Please delete options that are not relevant.

• [x] New feature (non-breaking change which adds functionality)

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation comments
• [x] My changes generate no new warnings

[ninjadev64]

Line 38 and 55, could they just be return background + firstMessage + lastMessage + cursor and not need another message variable, or is that not how mesh addition works?

[stackotter]

Line 38 and 55, could they just be return background + firstMessage + lastMessage + cursor and not need another message variable, or is that not how mesh addition works?

Yep, I didn't notice that while looking through the rest, the meshes for the message should only be generated once.