replace glyphs in material-nf which are overwritten arabic chars

NerdFonts has one big issue. They overwrite code points which are
reserved for chinese and arabic letters (range 0xf500-0xfd46).
While overwriting chinese letters is bad, they are LTR glyphs and print
"normally".
`net_loopback` and `update` used to be symbols that usually map
to arabic letters which make pango print those strings RTL. I
tried to fix this with the LTR marker 0x200e in greshake#1206 but it failed.
This PR replaces the symbols with similar ones that don't map to
the arabic charset.
nerd-font issue: ryanoasis/nerd-fonts#365
fixes greshake#1206

files/icons/material-nf.toml

@@ -48,7 +48,7 @@ music_play = "\uf909" # nf-mdi-play
 music_prev = "\uf9ad" # nf-mdi-skip_previous
 net_bridge = "\uf9a9" # nf-mdi-sitemap
 net_down = "\uf6d9" # nf-mdi-download
-net_loopback = "\ufbe9" # nf-mdi-loop
+net_loopback = "\uf56e" # nf-mdi-backup_restore
 net_modem = "\uf8f1" # nf-mdi-phone
 net_up = "\ufa51" # nf-mdi-upload
 net_vpn = "\ufa81" # nf-mdi-vpn
@@ -70,7 +70,7 @@ time = "\uf64f" # nf-mdi-clock
 toggle_off = "\ufa21" # nf-mdi-toggle_switch_off
 toggle_on = "\ufa20" # nf-mdi-toggle_switch
 unknown = "\uf685" # nf-mdi-comment_question_outline | TODO: Make default?
-update = "\ufbae" # nf-mdi-update
+update = "\uf8d4" # nf-mdi-package_up
 uptime = "\uf652" # nf-mdi-clock_in
 volume_empty = "\ufa7e" # nf-mdi-volume_low
 volume_full = "\ufa7d" # nf-mdi-volume_high