Use IBM Plex Sans / Lilex (#36084)

The Zed Plex fonts were found to violate the OFL by using the word Plex
in the name.

Lilex has better ligatures and box-drawing characters than Zed Plex
Mono, but Zed Plex Sans should be identical
to IBM Plex Sans.

Closes #15542
Closes zed-industries/zed-fonts#31

Release Notes:

- The "Zed Plex Sans" and "Zed Plex Mono" fonts have been replaced with
"IBM Plex Sans" and "Lilex". The old names still work for backward
compatibility. Other than fixing line-drawing characters, and improving
the ligatures, there should be little visual change as the fonts are all
of the same family.
- Introduced ".ZedSans" and ".ZedMono" as aliases to allow us to easily
change the default fonts in the future. These currently default to "IBM
Plex Sans" and "Lilex" respectively.

nix/shell.nix

@@ -46,8 +46,8 @@
       # outside the nix store instead of to `$src`
       FONTCONFIG_FILE = makeFontsConf {
         fontDirectories = [
-          "./assets/fonts/plex-mono"
-          "./assets/fonts/plex-sans"
+          "./assets/fonts/lilex"
+          "./assets/fonts/ibm-plex-sans"
         ];
       };
       PROTOC = "${protobuf}/bin/protoc";