Increase default `line-spacing' to 0.05, 0.10 or 0.15 [proposal]

[Stefan Kangas]

In typography, it is well known that the line height (leading) should be
a bit larger than the font size to improve legibility.  The drawback of
a larger leading is that fewer lines of text are visible on the screen
at the same time.

Emacs by default uses a leading of the font size + a few pixels (the
`line-spacing' variable is 0).  Conventional wisdom seems to suggest
instead that 1.2em (where 1em is the font size) is often a good
choice.[1]  Other editors also tend to use a larger leading than us.[2]

I suggest that we increase the default value of `line-spacing' to
improve legibility.  I believe this would improve our OOTB experience
for the majority users and use-cases.  In cases where it doesn't, users
are still free to revert to whatever was there before.

Below are the three options that I suggest we consider.  Note that "If
[the] value [of `line-spacing'] is a floating point number, it specifies
the spacing relative to the default frame line height."

[I've included line numbers below for the three options on my screen and
font size, but obviously YMMV.]

A. Ideally, we would use 0.15 for maximum legibility.  This is close to
   1.2 em. [53 lines, 11.6 % fewer lines]

B. We could also consider 0.10 to be more cautious. [56 lines, 6.66 %]

C. A very conservative option is to use 0.05, which would be a small
   incremental improvement on what we already have. [58 lines, 3.33 %]

D. The default. [60 lines, 0.0 %]

All in all, I think option B would be best, as that means we lose very
few lines on the screen (6 %).  Alternatively, we could pick option C,
which loses even fewer (3 %).

The way I suggest we carry this out is an experiment on master for 30
days, similarly to how we recently did with unbinding `M-o'.  This would
allow us to gather feedback and see how well this works in practice.

Footnotes:
[1] See also this link for reference:
    
https://ux.stackexchange.com/questions/35270/is-there-an-optimal-font-size-line-height-ratio

[2]  See screenshots from some common editors here:

     VSCode
     
https://user-images.githubusercontent.com/1487073/58344409-70473b80-7e0a-11e9-8570-b2efc6f8fa44.png

     Atom
     
https://upload.wikimedia.org/wikipedia/commons/5/5f/Atom_screenshot_v1.41.0.png

     GEdit
     https://upload.wikimedia.org/wikipedia/commons/3/3f/Gedit.png