add tsc problem matcher for GH Actions ...

that detects the correct line number.
This gives us nice annotations if any `tsc` errors occur in a PR, which
helps track down issues.
`actions/setup-node` already registers a `tsc` problem matcher, but
theirs doesn't respect line numbers
(actions/setup-node#97). Once that issue is
fixed we can revert this commit.

.github/problemMatchers/tsc.json

@@ -0,0 +1,19 @@
+{
+    "problemMatcher": [
+        {
+            "owner": "tsc",
+            "pattern": [
+                {
+                    "name": "tsc",
+                    "regexp": "^([^\\s].*)[\\(:](\\d+)[,:](\\d+)(?:\\):\\s+|\\s+-\\s+)(error|warning|info)\\s+TS(\\d+)\\s*:\\s*(.*)$",
+                    "file": 1,
+                    "line": 2,
+                    "column": 3,
+                    "severity": 4,
+                    "code": 5,
+                    "message": 6
+                }
+            ]
+        }
+    ]
+}

.github/workflows/ci.yml

@@ -44,6 +44,9 @@ jobs:
             - name: Install dependencies
               run: yarn --frozen-lockfile
 
+            - name: Register Problem Matcher
+              run: echo "##[add-matcher].github/problemMatchers/tsc.json"
+
             - name: Run tests & checks
               run: yarn testcheck