add two editor variables to line-numbers

add two editor variables to line-numbers, `line-format` and `custom-current-line`. they allow the user to set the line format and choose a custom current line if they want.

for example :

`(setf (variable-value 'line-number-format :global) "~2D ")`

for a narrow line-number column. and

`(setf (variable-value 'custom-current-line :global) "->")`

for a current-line indicator of `->`

src/ext/line-numbers.lisp

@@ -20,11 +20,11 @@
 
 (define-editor-variable line-number-format "~6D "
   "Set to desired format, for example, \"~2D \" for a
-two-character line-number column.")
+two-character line-number column and a margin of one space.")
 
 (define-editor-variable custom-current-line nil
   "Set to desired current-line value when relative line
-numbers are active, for example, \"->\".  NIL will make the
+numbers are active, for example, \"->\".  NIL will make
 the absolute value of the current line display.")
 
 (define-attribute line-numbers-attribute