Add show_src_lines_frame option

[HeyNonster]

This addresses the feedback from #1094 and resolves #1135.

Description

Both up and down only show 1 source line when called, I think it would be more useful if there was a configurable way to provide more context.

This adds a show_src_lines_frame (happy to rename this) configuration option (which defaults to 1) that determines how many source lines get shown for frame operations up, down, and set (frame).

I think the lib/debug/config.rb changes are easier to review without whitespace:

git diff -w lib/debug/config.rb
diff --git a/lib/debug/config.rb b/lib/debug/config.rb
--- a/lib/debug/config.rb
+++ b/lib/debug/config.rb
@@ -14,6 +14,7 @@ module DEBUGGER__
     # UI setting
     log_level:           ['RUBY_DEBUG_LOG_LEVEL',      "UI: Log level same as Logger",               :loglevel, "WARN"],
     show_src_lines:      ['RUBY_DEBUG_SHOW_SRC_LINES', "UI: Show n lines source code on breakpoint", :int, "10"],
+    show_src_lines_frame:['RUBY_DEBUG_SHOW_SRC_LINES_FRAME', "UI: Show n lines source code on frame operations", :int, "1"],
     show_evaledsrc:      ['RUBY_DEBUG_SHOW_EVALEDSRC', "UI: Show actually evaluated source",         :bool, "false"],
     show_frames:         ['RUBY_DEBUG_SHOW_FRAMES',    "UI: Show n frames on breakpoint",            :int, "2"],
     use_short_path:      ['RUBY_DEBUG_USE_SHORT_PATH', "UI: Show shorten PATH (like $(Gem)/foo.rb)", :bool, "false"],

Also probably useful to note that byebug displays more lines by default making it quite a difference switching from byebug to debug:

byebug-vs-debug.mov

[HeyNonster]

@ko1, please let me know if you would like me to change the name of the configuration!

[HeyNonster]

Closes #1135.

[ko1]

Thank you!