jshell tool: Edit Pad has readability issues

From Paul Deitel --

a. Use a fixed-width code font for readability

b. Use a bigger font size or provide controls for changing the font size--the current size is tiny and hard to read

c. Add 5-10 pixels of padding between the textbox containing the code and the window border—right now the left edge of the code is right up against the window’s border, again making the code hard to read.

d. Separate each snippet from the next by a blank line.