suggestion: add `html()` and `css()` tagged template functions

[iuioiua]

@std/http/file-server already uses a html() function that simply returns the passed in arguments as strings but enabled formatting thanks to improvements to deno fmt. I suggest we factor that out somewhere in this codebase, as well as a css(). Maybe @std/http/syntax or @std/html/syntax.

Tests

import { assertEquals } from "@std/assert";
import { html } from "@std/html/syntax";

Deno.test("html returns raw interpolated string", () => {
  const name = "world";
  const result = html`
    <p>Hello, ${name}!</p>
  `;
  assertEquals(
    result,
    `
    <p>Hello, world!</p>
  `,
  );
});

Deno.test("html handles multiple interpolations", () => {
  const a = "foo";
  const b = "bar";
  const result = html`
    <p>A: ${a}, B: ${b}</p>
  `;
  assertEquals(
    result,
    `
    <p>A: foo, B: bar</p>
  `,
  );
});

Deno.test("html handles no interpolations", () => {
  const result = html`
    <p>Just a string</p>
  `;
  assertEquals(
    result,
    `
    <p>Just a string</p>
  `,
  );
});

Deno.test("html handles special characters", () => {
  const special = "<>&\"'";
  const result = html`
    <p>Special: ${special}</p>
  `;
  assertEquals(
    result,
    `
    <p>Special: <>&\"'</p>
  `,
  );
});

Deno.test("html preserves raw backslashes and escapes", () => {
  const path = "\\foo\\bar";
  const result = html`
    <p>Path: ${path}</p>
  `;
  assertEquals(
    result,
    `
    <p>Path: \\foo\\bar</p>
  `,
  );
});

[lionel-rowe]

Deno.test("html handles special characters", () => {
  const special = "<>&\"'";
  const result = html`
    <p>Special: ${special}</p>
  `;
  assertEquals(
    result,
    `
    <p>Special: <>&\"'</p>
  `,
  );
});

That's definitely not how most such functions work, as it'd be an XSS vector given untrusted input. Usually, the logic is something like this:

const raw = "<>&\"'";
const trusted = html`<>&"'`;

// not strictly necessary, as it doesn't necessarily have to be a class, but it can't be a `string`,
// as there would be no way to indicate if it was trusted or not.
assertInstanceOf(trusted, Html);

assertEquals(trusted.toString(), raw);
assertEquals(html`${trusted}`.toString(), raw);
assertEquals(html`${raw}`.toString(), "&lt;&gt;&amp;&quot;&#39;");