[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_set.test.ts

@@ -54,6 +54,21 @@ describe("t-set", () => {
     expect(renderToString(template)).toBe("ok");
   });
 
+  test("body with backslash at top level", () => {
+    const template = '<t t-set="value">\\</t><t t-esc="value"/>';
+    expect(renderToString(template)).toBe("\\");
+  });
+
+  test("body with backtick at top-level", () => {
+    const template = '<t t-set="value">`</t><t t-esc="value"/>';
+    expect(renderToString(template)).toBe("`");
+  });
+
+  test("body with interpolation sigil at top level", () => {
+    const template = '<t t-set="value">${very cool}</t><t t-esc="value"/>';
+    expect(renderToString(template)).toBe("${very cool}");
+  });
+
   test("set from body literal (with t-if/t-else", () => {
     const template = `
       <t>