feature: custom series, add "extra" in el options for users to config their own properties for animation.

fix: little modify in during api: add return this to setter and change name from "setAttr" to "setTransform".

fix: fix custom series merge children strategy (force no empty children even thought merging children).

And add rule "no-var" to eslintrc

Use eslint rule to check the diff between var and let:
    "no-use-before-define": "off"
    "@typescript-eslint/no-use-before-define": 2
    no-redeclare: 2
    block-scoped-var: 2
    no-loop-func: 2
(2) By the way add some eslint rule and unify some code style.

Note: lots of the common type are put in `src/util/types.ts`.

(1) On value axis, the auto-calculated bar width might be near 0, which make the bar invisible.
So we add option `barMinWidth` (default 1 in category and default null in other coord sys).
(2) The fix #5316 (commit 2823ab50) seems not work any more currently.
check the cases in `echarts/test/bar-width.html`. This fix make it works.

feat: provide `echarts.registerShape` and `echarts.getShapeClass` for custom shape in `custom series` and `graphic component`.

fix(custom series): the hover style should be always set but should not cache in hover state, otherwise the previous cache will be not reset and the previous hover style will remain; fix #11103.