Migrate final development VSIX extension to extensions.json

- Richterger Perl language server now published on the Open VSX registry
  (see richterger/Perl-LanguageServer#98,
  which is now closed)

Dockerfile

@@ -261,7 +261,8 @@ ARG HOME=/home/newsnow
 ################################################################################
 # INSTALL DEVELOPMENT VSIX PLUGINS
 #
-COPY --from=vsix-plugins --chown=$USER:$USER /root/theia-plugins $HOME/theia-plugins/
+# (disabled as there are currently no VSIX extensions needing to be embedded in the image)
+# COPY --from=vsix-plugins --chown=$USER:$USER /root/theia-plugins $HOME/theia-plugins/
 
 ################################################################################
 # THEIA INTEGRATION

build/development/extensions.json

@@ -2,6 +2,7 @@
 	// See https://go.microsoft.com/fwlink/?LinkId=827846
 	// for the documentation about the extensions.json format
 	"recommendations": [
+      "richterger.perl",
       "vscode.css",
       "vscode.docker",
 		"dbaeumer.vscode-eslint",

build/development/install-vsix.sh

@@ -12,8 +12,8 @@ mkdir -p $PLUGIN_DIR
 #   curl --fail --silent --location --retry 3 --max-time 20 -o $PLUGIN_DIR/$file $vsix
 # done
 
-RICHTERGER="2.5.0"
-echo "Downloading vsix plugin richterger.perl-$RICHTERGER.vsix ..." >&2
-curl --fail --silent --location --retry 2 --max-time 20 -o $PLUGIN_DIR/richterger.perl-$RICHTERGER.vsix https://marketplace.visualstudio.com/_apis/public/gallery/publishers/richterger/vsextensions/perl/$RICHTERGER/vspackage --compressed || \
-curl --fail --silent --location --retry 3 --max-time 20 -o $PLUGIN_DIR/richterger.perl-$RICHTERGER.vsix https://storage.googleapis.com/dockside/vsix/richterger.perl-$RICHTERGER.vsix || \
-curl --fail --silent --location --retry 3 --max-time 20 -o $PLUGIN_DIR/richterger.perl-2.2.0.vsix https://storage.googleapis.com/dockside/vsix/richterger.perl-2.2.0.vsix
+# RICHTERGER="2.5.0"
+# echo "Downloading vsix plugin richterger.perl-$RICHTERGER.vsix ..." >&2
+# curl --fail --silent --location --retry 2 --max-time 20 -o $PLUGIN_DIR/richterger.perl-$RICHTERGER.vsix https://marketplace.visualstudio.com/_apis/public/gallery/publishers/richterger/vsextensions/perl/$RICHTERGER/vspackage --compressed || \
+# curl --fail --silent --location --retry 3 --max-time 20 -o $PLUGIN_DIR/richterger.perl-$RICHTERGER.vsix https://storage.googleapis.com/dockside/vsix/richterger.perl-$RICHTERGER.vsix || \
+# curl --fail --silent --location --retry 3 --max-time 20 -o $PLUGIN_DIR/richterger.perl-2.2.0.vsix https://storage.googleapis.com/dockside/vsix/richterger.perl-2.2.0.vsix