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.

assets/fonts/lilex/OFL.txt

@@ -1,8 +1,9 @@
-Copyright © 2017 IBM Corp. with Reserved Font Name "Plex"
+Copyright 2019 The Lilex Project Authors (https://github.com/mishamyrt/Lilex)
 
 This Font Software is licensed under the SIL Open Font License, Version 1.1.
 This license is copied below, and is also available with a FAQ at:
-http://scripts.sil.org/OFL
+https://scripts.sil.org/OFL
+
 
 -----------------------------------------------------------
 SIL OPEN FONT LICENSE Version 1.1 - 26 February 2007
@@ -89,4 +90,4 @@ COPYRIGHT HOLDER BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
 INCLUDING ANY GENERAL, SPECIAL, INDIRECT, INCIDENTAL, OR CONSEQUENTIAL
 DAMAGES, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
 FROM, OUT OF THE USE OR INABILITY TO USE THE FONT SOFTWARE OR FROM
-OTHER DEALINGS IN THE FONT SOFTWARE.
+OTHER DEALINGS IN THE FONT SOFTWARE.