This is experimental.

LuaTeX seems to have come a long way: it now uses harfbuzz for glyph shaping, and most microtype features seem to work with it now. We could consider switching from pdfTeX to LuaTeX.

I see two main options:

1. Just switch to LuaTeX but leave lmodern/fontenc as is. This provides a very similar result, but also doesn't give us much benefit.
2. Switch to LuaTeX, and use fontspec and the New Computer Modern family (which I believe is derived from Latin modern). This is what this PR does.

Pros:

• Better unicode support, access to more scripts and glyphs.
• Better indexing (@godbyk, is that true?).
• Potential access to more fonts, should we care.

Cons:

• Slower.
• New Computer Modern doesn't have nearly as many optical sizes as Latin Modern.
• Despite microtype claiming support, protrusions seem less sophisticated than with pdfTeX.
• Using NewCM for maths requires the unicode-math package, which is still advertised as "experimental".
• Hard to be sure if everything is still OK.

If you'd like to weigh in, it would be very useful if you could review the resulting PDF and see if you notice any major defects (e.g. look at some areas that you might recall as being typographically tricky in the past).

We don't have to make a decision any time soon, but it might be worth to start looking into this.