[echarts] Initialize echarts with built in dark/light theme matching LogSeq system settings

[swarminglogic]

To my knowledge, echarts doesn't have a way to set/chose a predefined theme, except at init, with something like

var chart = echarts.init(dom, 'dark');

This makes renders difficult to read, without overriding individual colors (which is both verbose and tedious). The example here https://github.com/xyhp915/logseq-fenced-code-plus/pull/19

Might instead look like:

I imagine the change would be something akin to https://github.com/xyhp915/logseq-fenced-code-plus/pull/18

But in https://github.com/xyhp915/logseq-fenced-code-plus/blob/81632545234de24969fa224312d7f9d4f8436620/src/echarts.tsx#L47

Lastly, Thanks for a great plugin.