Add 'clear', 'ls', and 'history' commands to shell

[PrachiJha-404]

###Summary

Adds support for the following shell commands:

• clear: Clears the screen
• ls: Lists available commands
• history: Prints previous inputs from the current session

###Implementation Details

• Added cmdClear, cmdLs, and cmdHistory functions
• Added history and history_count in the shell struct
• Added constant MAX_HISTORY

###Note

• Most of these are dummy functions, the rest of the implementations will be done after adding a filesystem.
• Minor updates were made to build.zig and .gitignore for local environment, these should not be pulled in the main repository