fix: don't double backslash escapes in non-CSS embedded templates

[bartlomieju]

Problem

When formatting embedded code in tagged templates, lone escape sequences get
their backslash doubled, which changes the meaning of the user's code:

html`\n`;                          // \n  ->  \\n
const s = html`<script>const d = fetch(\`\`);</script>`;  // \`  ->  \\`

Root cause

maybe_gen_tagged_tpl_with_external_formatter unescapes \\ -> \ before
calling the external formatter and then re-escapes every \ -> \\
afterwards. The blanket post-doubling also doubles lone escapes (\n, \`)
that were never part of a \\ pair.

The \\ -> \ collapse is genuinely needed for CSS: the CSS parser cannot
parse the double-backslash form of escapes (e.g. Tailwind
.bg-\\[\\#86efac\\] errors; it needs .bg-\[\#86efac\]). But it must not be
applied to other embedded languages.

Fix

Gate the \\ <-> \ round-trip to embedded_lang == "css". Other languages
(html / xml / svg / sql) pass the raw text through verbatim, preserving their
escapes. The existing CSS \\ (Tailwind) behavior is unchanged.

Test

Added a regression spec to tests/specs/external_formatter/html.txt asserting
html`\n` stays \n. Verified it fails before the fix and passes after; the
existing CSS \\ spec still passes.

---

This PR was authored and opened with AI assistance (Claude Code). The fix was
manually verified end-to-end against Deno's formatter (which embeds this plugin):
it resolves denoland/deno#30103 and denoland/deno#30948, with Deno's full fmt
spec suite passing.