[Enhancement] Add a method to force Continue to include file(s) in the index

[sealad886]

Validations

• [X] I believe this is a way to improve. I'll try to join the Continue Discord for questions
• [X] I'm not able to find an open issue that requests the same enhancement

Problem

Continue indexing uses .gitignore to determine files that should not be in the index. There is currently no way to force the index to include a file or directory that is listed in .gitignore.

Solution

Create a .continueinclude file that is checked after ignore to add missing files and directories back into the index.