Highlight references to line numbers using the line number color

[hackerb9]

One feature Telnet23's highlighting has that basic-mode.el should steal acquire is highlighting line numbers when they are referenced by statements like GOTO 10 in the line number color.

Note that Telnet23's regexps only handle numbers directly after THEN, ELSE, GO\s*TO, and GOSUB. There are other ways that BASIC can refer to line numbers.

These should all have the numbers highlighted as line numbers:

• ON var GOSUB 10, 20, 30
• ON var GO TO 10, 20, 30
• RESTORE 10
• RESUME 10
• RETURN 10
• RUN 10
• LIST -500
• LLIST 10
• RENUM 1000,10
• DELETE 10-40
• IF ERL=10 THEN...

Exceptions:

• RENUM [newnum] [,[oldnum] [,increment]]
  Increment should not be highlighted.

Originally posted by @hackerb9 in #21 (comment)