echarts: simplify usage and remove current limitations

[mij]

Hello folks,

The patch attached resolves, in a fully backwards-compatible way, the problem of echarts in LoveIt unable to support Javascript.

The patch supports this new form of using the shortcode:

{{< echarts file="charts/mychart.js" >}}{{< /echarts >}}

The shortcode expands this to a new attribute

<div class="echarts" ... data-filename="charts/mychart.js" ...>

and when the page is loaded, theme.js automatically recognizes which divs to load as a JS module vs with embedded specification, based on the presence of that data-filename attribute.

The documentation is updated accordingly, to list both ways of use: the "embedded" way (current), and the "referenced" way (with JS module).

As charts become non-trivial, the embedded way quickly become unusable, not only because it prevents using JavaScript which is essential to non-trivial charts, but also because the effort of converting specs back and forth from JavaScript to JSON/YAML/TOML when prototyping the chart with echart's editor becomes huge.

There are additional benefits of this approach:

• Cleaner pages, not overbloating posts with specs code. Also doesn't break syntax highlighting of some browsers.
• Trivially allows re-using a chart in multiple posts.
• Improves speed of page loads by removing echarts code and allowing browser to cache JS file with it.

Nota bene: I haven't committed the auto-generate assets/js/theme.js.

Let me know if there's any additional adaptation you'd like to see in this.

[sonarcloud]

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[mij]

Hello folks!

Is there any more input or code I could provide to encourage merging this patch?

A brief memo:

• Fully backwards compatible.
• Adds description of new feature in documentation.
• Enables major functionality in using echarts, towards becoming must-have as echarts relies more on JavaScript expressions over time.

Thanks!