*/*: Use [[ ... ]] instead of POSIX [ ... ]

as per the coding style guide:
https://projects.gentoo.org/qa/policy-guide/ebuild-format.html#pg0101

only these changes were done:

[ -e ... ]    to  [[ -e ... ]]
[ -z ... ]    to  [[ -z ... ]]
[ -n ... ]    to  [[ -n ... ]]
[ ! -f ... ]  to  [[ ! -f ... ]]
[ -f ... ]    to  [[ -f ... ]]
[ -d ... ]    to  [[ -d ... ]]

the changes were generated using this command:

```
find -name '*.ebuild' -exec sed -i -E \
's|([^[])\[+(\s+)([^]]+)(\s+)\]+([^]]\|$)|\1\[\[\2\3\4\]\]\5|g' \
{} \+
```

this is aimed to be a series of commits meant to replace all instance of
POSIX [ ... ] to bash's [[ ... ]], to make guru up to date with the
coding style guide.

this process is divided up into a series of commits in order to make
review and testing more easy

Signed-off-by: ingenarel (NeoJesus) <ingenarel_neojesus@disroot.org>
Part-of: https://codeberg.org/gentoo/guru/pulls/6
Signed-off-by: Michał Górny <mgorny@gentoo.org>

x11-themes/catppuccin-cursors/catppuccin-cursors-1.0.2.ebuild

@@ -92,7 +92,7 @@ BDEPEND="app-arch/unzip"
 src_install() {
 	insinto "/usr/share/icons"
 	for folder in * ; do
-		if [ -d "${folder}" ]; then
+		if [[ -d "${folder}" ]]; then
 			doins -r "${folder}"
 		fi
 	done

x11-themes/catppuccin-cursors/catppuccin-cursors-2.0.0.ebuild

@@ -31,7 +31,7 @@ BDEPEND="app-arch/unzip"
 src_install() {
 	insinto "/usr/share/icons"
 	for folder in * ; do
-		if [ -d "${folder}" ]; then
+		if [[ -d "${folder}" ]]; then
 			doins -r "${folder}"
 		fi
 	done

x11-themes/catppuccin-gtk/catppuccin-gtk-0.7.5.ebuild

@@ -94,7 +94,7 @@ BDEPEND="
 src_install() {
 	insinto "/usr/share/themes"
 	for folder in * ; do
-		if [ -d "${folder}" ]; then
+		if [[ -d "${folder}" ]]; then
 			doins -r "${folder}"
 		fi
 	done

x11-themes/catppuccin-gtk/catppuccin-gtk-1.0.3.ebuild

@@ -94,7 +94,7 @@ BDEPEND="
 src_install() {
 	insinto "/usr/share/themes"
 	for folder in * ; do
-		if [ -d "${folder}" ]; then
+		if [[ -d "${folder}" ]]; then
 			doins -r "${folder}"
 		fi
 	done