Use the FabricClientCommandSource to replace calls to other methods

[xpple]

Closes #407. This PR replaces most occurrences of MinecraftClient#getInstance with FabricClientCommandSource#getClient and replaces ClientCommandHelper#sendFeedback and ClientCommandHelper#sendError with FabricClientCommandSource#sendFeedback and FabricClientCommandSource#sendError respectively. Where applicable, access of the player or world through the client instance have been replaced with the according methods of FabricClientCommandSource. In some occasions it wasn't possible or wouldn't have been beneficial towards the readability of the code to replace a call to MinecraftClient#getInstance (for example FindItemCommand and MoteCommand), so I didn't apply any changes there.