[FIX] compiler: correctly escape special characters in template literals

Previously, there were a few places where the compiler would create
strings from template content and emit them as template literals, but
didn't properly escape characters or character sequences with special
meanings, in particular: backslashes, backticks, and interpolation
sigils.

This commit fixes this in:
- block creation (interpolation sigils were not escaped)
- text node creation (no escaping was performed)
- comment not creation (no escaping was performed)
- default values for t-esc (no escaping was performed)
- body of a t-set (no escaping was performed)

tests/compiler/t_esc.test.ts

@@ -121,4 +121,19 @@ describe("t-esc", () => {
     mount(bdom, fixture);
     expect(fixture.querySelector("span")!.textContent).toBe("<p>escaped</p>");
   });
+
+  test("default with backslash at top level", () => {
+    const template = '<t t-esc="undefined">\\</t>';
+    expect(renderToString(template)).toBe("\\");
+  });
+
+  test("default with backtick at top-level", () => {
+    const template = '<t t-esc="undefined">`</t>';
+    expect(renderToString(template)).toBe("`");
+  });
+
+  test("default with interpolation sigil at top level", () => {
+    const template = '<t t-esc="undefined">${very cool}</t>';
+    expect(renderToString(template)).toBe("${very cool}");
+  });
 });