IMPORT should be direct mode only

[dansanderson]

Test Environment (required)
You can use MEGA65INFO to retrieve this.

• ROM Release: 920409

Describe the bug
The IMPORT command modifies the current BASIC program and should therefore only be a direct mode command. However, as of ROM 920409, it lacks a check for direct mode and does attempt to modify the currently running BASIC program if the program contains an IMPORT statement. This almost works but has undefined behavior. We should add the direct mode check and error to the beginning of this command.

(Historically, IMPORT has had very poor error reporting. We only recently fixed it to report disk errors.)