tools: Filter "highlight" case insensitive

There was a problem with a ‘highlight’ that was misunderstood as a
spelling mistake in lib-wally. Since ‘hightlight’ is already filtered
out, we simply instruct grep to ignore upper/lower case when filtering.

Signed-off-by: Peter Neuroth <[email protected]>

Author: nepet

tools/check-spelling.sh

@@ -1,6 +1,6 @@
 #!/usr/bin/env bash
 
-if git --no-pager grep -nHiE 'l[ightn]{6}g|l[ightn]{8}g|ilghtning|lgihtning|lihgtning|ligthning|lighnting|lightinng|lightnnig|lightnign' -- . ':!tools/check-spelling.sh' ':!tests/data/routing_gossip_store' | grep -vE "highlighting|LightningGrpc"; then
+if git --no-pager grep -nHiE 'l[ightn]{6}g|l[ightn]{8}g|ilghtning|lgihtning|lihgtning|ligthning|lighnting|lightinng|lightnnig|lightnign' -- . ':!tools/check-spelling.sh' ':!tests/data/routing_gossip_store' | grep -viE "highlighting|LightningGrpc"; then
     echo "Identified a likely misspelling of the word \"lightning\" (see above). Please fix."
     echo "Is this warning incorrect? Please teach tools/check-spelling.sh about the exciting new word."
     exit 1