Add an `unremovable` outfit attribute

Problem Description

There is no clean way to have an outfit that can't be removed from a ship by the player but can be taken out by the game.

Related Issue Links

#1412 asks for such an attribute, among other things, closed by the author for lack of interest. #2372 also asks for this attribute, but was closed as completed by MZ without any explanation.

Desired Solution

Add a new outfit tag, unremovable, that prevents the player from uninstalling, selling, or in any way removing an outfit from the ship it is currently installed on. Implies unplunderable.

Alternative Approaches

In the particular use case I'm thinking of, adjusting attribute checks so that a mission can take an outfit that causes a ship attribute to go negative would also work.

Additional Context

No response