templates: Enable Handlebars strict mode.

https://handlebarsjs.com/api-reference/compilation.html

Signed-off-by: Anders Kaseorg <anders@zulip.com>

web/tests/lib/handlebars.js

@@ -24,7 +24,7 @@ hb.JavaScriptCompiler = ZJavaScriptCompiler;
 
 function compile_hbs(module, filename) {
     const code = fs.readFileSync(filename, "utf8");
-    const pc = hb.precompile(code, {preventIndent: true, srcName: filename});
+    const pc = hb.precompile(code, {preventIndent: true, srcName: filename, strict: true});
     const node = new SourceNode();
     node.add([
         'const Handlebars = require("handlebars/runtime");\n',

web/webpack.config.ts

@@ -146,6 +146,7 @@ export default (
                             "rendered_markdown",
                             "tooltip_hotkey_hints",
                         ],
+                        precompileOptions: {strict: true},
                         preventIndent: true,
                         // This replaces relative image resources with
                         // a computed require() path to them, so their