.vscode/tasks.json: Add build tasks for Bootstrap and Build with B2 (#12

)

This change complements the build instructions in the CONTRIBUTING.adoc,
where it is explained to bootstrap the 'stable' engine to
create b2 at the root.

Then, run one of the

   two ways to build this be engine.
   You can either (a) run b2 b2 at the root,
   or (b) run build.sh/bat in src/engine.

.vscode/tasks.json

@@ -4,7 +4,39 @@
     "version": "2.0.0",
     "tasks": [
         {
-            "label": "B2 Engine Build",
+            "label": "B2 Engine Bootstrap",
+            "type": "shell",
+            "windows": {
+                "command": "bootstrap.bat"
+            },
+            "osx": {
+                "command": "./bootstrap.sh"
+            },
+            "linux": {
+                "command": "./bootstrap.sh"
+            },
+            "options": {
+                "cwd": "${workspaceFolder}"
+            },
+            "problemMatcher": {
+                "owner": "cpptools",
+                "fileLocation": [
+                    "relative",
+                    "${workspaceFolder}/src/engine"
+                ],
+                "pattern": {
+                    "regexp": "^(.*):(\\d+):(\\d+):\\s+(warning|error):\\s+(.*)$",
+                    "file": 1,
+                    "line": 2,
+                    "column": 3,
+                    "severity": 4,
+                    "message": 5
+                }
+            },
+            "group": "build"
+        },
+        {
+            "label": "B2 Engine Build (using build.sh|bat)",
             "type": "shell",
             "windows": {
                 "command": "build.bat"
@@ -35,6 +67,38 @@
             },
             "group": "build"
         },
+        {
+            "label": "B2 Engine Build (using B2)",
+            "type": "shell",
+            "windows": {
+                "command": "${workspaceFolder}\\b2.exe b2"
+            },
+            "osx": {
+                "command": "${workspaceFolder}/b2 b2"
+            },
+            "linux": {
+                "command": "${workspaceFolder}/b2 b2"
+            },
+            "options": {
+                "cwd": "${workspaceFolder}"
+            },
+            "problemMatcher": {
+                "owner": "cpptools",
+                "fileLocation": [
+                    "relative",
+                    "${workspaceFolder}"
+                ],
+                "pattern": {
+                    "regexp": "^(.*):(\\d+):(\\d+):\\s+(warning|error):\\s+(.*)$",
+                    "file": 1,
+                    "line": 2,
+                    "column": 3,
+                    "severity": 4,
+                    "message": 5
+                }
+            },
+            "group": "build"
+        },
         {
             "label": "B2 Engine Infer",
             "type": "shell",