I was going to upgrade to the unicode 15 font, but in testing this I
decided that the logic is slightly complex and the glyphs are often
difficult to see at most terminal font sizes, which generates questions
from users, so just fall back to notdef.
Built using:
```
./FontForge-2020-11-07-21ad4a1-x86_64.AppImage --script $PWD/font-patcher "$PWD/src/unpatched-fonts/NerdFontsSymbolsOnly/NerdFontsSymbolsOnly Template 1000 em.ttf" --powerline --use-single-width-glyphs -out /tmp/nerd-fonts-out --fontawesome --fontawesomeextension --fontlinux --octicons --codicons --powersymbols --powerline --powerlineextra --mdi --weathericons
```
which is everything *except* Pomicons at the time of writing, pending
clarifications of its distribution license
(https://github.com/ryanoasis/nerd-fonts/issues/266)
refs: https://github.com/wez/wezterm/issues/1521
The default we use on macOS looks decent. Roboto is a similar
looking font that we can use for the other platforms.
I may make it the same on all three once I've had a chance
to compare it on a mac.
Bundle the *Last Resort High-Efficiency* font from
https://github.com/unicode-org/last-resort-font/
version 13.001 (Oct 22 2020).
This provides a more useful fallback glyph than we'd otherwise
produce if there is no matching glyph in any of the fonts.
Its license is OFL-1.1 which is compatible with the other
bundled fonts.
These serve two purposes:
* Provide a consistent default font for new installations,
that happens to show off ligature and color emoji support
out of the box.
* Provide a reasonable fallback in case the configuration is broken
Both fonts are distributed under the terms of the OFL 1.1.
refs: https://github.com/wez/wezterm/issues/263
