feat: support hidden lines in example code blocks

[crowlbot]

Implements hideable lines in example code blocks, à la rustdoc's documentation tests.

Closes denoland/deno#21574.

Behavior

Within a JavaScript/TypeScript example code block:

• A line beginning with # (hash + space), or consisting solely of #, is hidden from the rendered documentation.
• A line beginning with ## is an escape and renders as a single leading #.

# import { assertEquals } from "@std/assert";
const result = add(1, 2);
assertEquals(result, 3);

renders as:

const result = add(1, 2);
assertEquals(result, 3);

This lets authors keep boilerplate (imports, setup) runnable in doc tests (deno test --doc already strips these markers) without cluttering the rendered output.

Notes / design choices

• Hidden lines are removed from the displayed code in both the HTML renderer and the terminal printer. In the HTML output they are retained in the copy button's data-copy so copied snippets still execute as written.
• Processing is intentionally limited to JS/TS code-block languages (ts, js, tsx, mts, …). Other blocks (e.g. shell snippets that legitimately begin a line with #) are rendered verbatim. Happy to widen/narrow this set if preferred.
• Shebang lines (#!) and private fields (#x, no following space) are left untouched.

Tests

Added unit tests for the line-processing helper, the terminal printer, and the HTML renderer. Existing snapshot tests are unaffected. cargo test, cargo fmt -- --check, and cargo clippy --all-targets -- -D clippy::all all pass.

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