sh: Switch yay to paru

Removed outdated `syu` symlink which just hooks into topgrade.
Replaced it with simple function that tries for topgrade, paru, yay,
pacman, in that order. Can still be invoked with simple `syu` command,
but *only* through interactive terminal use.

Switched git pre-commit hook to default to paru instead of yay when
compiling installed package lists for dotfile commits.

.githooks/prepare-commit-msg

@@ -6,7 +6,7 @@ COMMIT_SOURCE="$2"
 BOOTSTRAPDIR="bootstrap"
 pkgfileloc="$(git rev-parse --show-toplevel)/$BOOTSTRAPDIR/packages.txt"
 pkgignoreloc="$(git rev-parse --show-toplevel)/$BOOTSTRAPDIR/packages_ignore.txt"
-listgen="yay"
+listgen="paru"
 
 err() {
   printf "\x1b[33mCAUTION:\x1b[0m %s\n" "$*"
@@ -18,7 +18,7 @@ if [ ! -f "$pkgfileloc" ]; then
 fi
 
 if ! type "$listgen" >/dev/null 2>&1; then
-  err "Yay not installed on machine, can not reliably determine package differences!"
+  err "List generator not installed on machine, can not reliably determine package differences!"
 fi
 
 # get commited packages, remove empty lines
@@ -30,7 +30,7 @@ sed -e '/^[#$]/d' <"$pkgfileloc" | sort >"$pkgcommited"
 # q removes extraneous info
 # e only lists explicitly installed
 # tt removes those that are depended on, but NOT those optionally depended on
-pkgcurrent=$(yay -Qqett | sort)
+pkgcurrent=$("$listgen" -Qqett | sort)
 # remove those listed in package_ignore.txt
 if [ -f "$pkgignoreloc" ]; then
   pkgcurrent=$(echo "$pkgcurrent" | comm -23 - "$pkgignoreloc")