More machine code generating words for ISR programming

[TG9541]

#382 adds words for relative addressing in IF .. ELSE .. THEN structures. This issue adds the following machine code generating words for coding ISR routines (since ISR programming works on the µC peripheral registers level and portability is a non-issue):

Word	example	code	comment
]B@IF	[ a b# ]B@IF	BTJF addr,#b,rel	test bit and execute IF clause if set
]C@IF	[ a ]C@IF	LD A,a JREQ rel	load a and and execute IF clause if not zero
]A<IF	[ c ]A<IF	CP A,#c JRPL rel	compare A with literal c, execute IF clause if A<c
]Y<IF	[ n ]Y<IF	CPW Y,#n JRPL rel	compare Y with literal n, execute IF clause if Y<n
]@	[ a ]@	-	read value from literal address a