ast.c: Rename "save_script_lines" to "keep_script_lines"

... as per ko1's preference. He is preparing to extend this feature to
ISeq for his new debugger. He prefers "keep" to "save" for this wording.
This API is internal and not included in any released version, so I
change it in advance.

lib/error_highlight/core_ext.rb

@@ -16,7 +16,7 @@ module ErrorHighlight
 
       loc = locs.first
       begin
-        node = RubyVM::AbstractSyntaxTree.of(loc, save_script_lines: true)
+        node = RubyVM::AbstractSyntaxTree.of(loc, keep_script_lines: true)
         opts = {}
 
         case self