fix: resolve grammar from leading token of code fence info string

[lostbean]

Problem

code.highlight looks up the entire code fence info string in the languages dictionary:

case dict.get(config.languages, language) {

When an author decorates a fence with a title or filename — a common convention — the info string no longer matches a bare language key, so highlighting silently falls back to unhighlighted text:

```cpp:main.ino      ← language is "cpp:main.ino", not "cpp" → Error(Nil)
```cpp main.ino      ← CommonMark: language is "cpp", rest is extra info → also missed

Repro

import blogatto/config/post/code

pub fn main() {
  let cfg = code.default()
  code.highlight(cfg, "cpp:main.ino", "int x = 1;")
  // before: Error(Nil) — block renders unhighlighted
  // after:  Ok([...highlighted spans...])
}

Fix

Match the full info string first (so explicitly registered language names — including any with unusual characters — keep resolving exactly as before), then fall back to the leading token: everything before the first space or :. An unknown leading token still returns Error(Nil), so behaviour for genuinely unsupported languages is unchanged.

The split is intentionally minimal — space and : only — matching CommonMark's "language is the text up to the first whitespace" rule plus the widespread lang:filename convention. No public API changes; the two new helpers are private.

Tests

Added to test/blogatto/config/post/code_test.gleam:

• cpp:main.ino (colon suffix) highlights
• cpp main.ino (space suffix) highlights
• bare cpp still highlights (no regression)
• brainfuck:hello.bf (unknown leading token) still returns Error(Nil)

I verified the two positive tests fail without the source change and pass with it. Full suite: 506 passed, no failures. gleam format --check src test is clean.

Downstream impact

In a blog built on Blogatto, this currently forces a workaround: re-tokenising code blocks ourselves with the trimmed language after Blogatto's lookup misses. With this fix the framework's own highlight wrapper handles decorated fences directly, letting that workaround be removed.

[cocogitto-bot]

✔️ 760ba3e - Conventional commits check succeeded.

[veeso]

lgtm.

Nice contribution, I didn't even know about that syntax for code blocks.

Thank you