[PHP] Enable Mbstring (#371)

Mbstring is such a popular PHP extension that it is used everywhere and most libraries don't work without it. It can be compiled directly with PHP.
The official Docker images of PHP include it.

src/php/devcontainer-feature.json

 {
     "id": "php",
-    "version": "1.0.9",
+    "version": "1.0.10",
     "name": "PHP",
     "documentationURL": "https://github.com/devcontainers/features/tree/main/src/php",
     "options": {

src/php/install.sh

@@ -169,7 +169,7 @@ install_php() {
         VERSION_CONFIG="--with-pear"
     fi
 
-    ./configure --prefix="${PHP_INSTALL_DIR}" --with-config-file-path="$PHP_INI_DIR" --with-config-file-scan-dir="$CONF_DIR" --enable-option-checking=fatal --with-curl --with-libedit --with-openssl --with-zlib --with-password-argon2 --with-sodium=shared "$VERSION_CONFIG" EXTENSION_DIR="$PHP_EXT_DIR";
+    ./configure --prefix="${PHP_INSTALL_DIR}" --with-config-file-path="$PHP_INI_DIR" --with-config-file-scan-dir="$CONF_DIR" --enable-option-checking=fatal --with-curl --with-libedit --enable-mbstring --with-openssl --with-zlib --with-password-argon2 --with-sodium=shared "$VERSION_CONFIG" EXTENSION_DIR="$PHP_EXT_DIR";
 
     make -j "$(nproc)"
     find -type f -name '*.a' -delete

test/php/test.sh

@@ -6,6 +6,7 @@ set -e
 source dev-container-features-test-lib
 
 check "PHP version" php --version
+check "Mbstring loaded" php -r "extension_loaded('mbstring') || throw new Error('Extension Mbstring is not loaded');"
 check "Composer version" composer --version
 
 # Report result