feat(markdown): add title support for code blocks (close #1277)

[nruffing]

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• Read the Contributing Guidelines.
• Provide a description in this PR that addresses what the PR is solving. If this PR is going to solve an existing issue, please reference the issue (e.g. close #123).

What is the purpose of this pull request?

• Bug fix
• New feature
• Documentation update
• Other

Description

Add support for being able to set a title for code blocks (e.g. a filename).

In case the data-ext attribute was being used for something else in a theme I added a new data-title attribute for the title and its value falls back to be the same as data-ext if an explicit title is not specified.

I opted to use enclose the title in < and > instead of square brackets so it would also easily work with the import code plugin which is already using square brackets.

Example

```ts <config/foo.ts> {1,2}:no-line-numbers
const foo = 'foo'

function bar () {
  return 1024
}
```

Tests have also been updated.

If we like this approach I can create a PR in theme-default to change the css to pull from the data-title attribute and update the docs.

closes #1277

Screenshots

Before

After

[meteorlxy]

Any reference of the [] and <> of other markdown extensions? I think it would be better to use a commonly used mark so that users could migrate from other framework to vuepress with ease (and vice versa 😅 )

[nruffing]

Here are some examples I found with some quick research. Based on this I could be on board with a key/value syntax like title="config/foo.ts".

fenceparser

```ts twoslash {1-3, 5} title="Hello, World"
```

MyST

```{code} yaml
:filename: myst.yml
project:
  title: Showing Filenames in code-blocks
```

Docusaurus

```jsx title="/src/components/HelloCodeTitle.js"
function HelloCodeTitle(props) {
  return <h1>Hello, {props.name}</h1>;
}
```

rehype-code-titles

```typescript:lib/mdx.ts
```

[meteorlxy]

Agree with the key-value implementation. It's also a more extendable syntax

[nruffing]

Sounds good. I will make those changes.