feat: new theme for ECharts 6.0

[Ovilia]

Brief Information

This pull request is in the type of:

• bug fixing
• new feature
• others

What does this PR do?

Fixed issues

Changes:

Legend

• Moved from top-center to be bottom-center, which helps avoid overlapping with the title

Title

• Moved from top-left to be top-center, to avoid failing to align with the left side of the grid

Grid

• containLabel changed to be true, to better place the axis when some axis labels are too long
• Adjusted left, right, top, bottom to give more empty space

Axis

• Hid axisTick to give a clearer look

Pie

• radius changed to be smaller to make more empty space
• labelLine.length2 changed to be larger to make it more elegant

Treemap

• Size changed from width: '80%' to be fixed left/right, which makes it more elegant when container is big
• breadcrumb.bottom changed from 0 to be 15

Document Info

One of the following should be checked.

• This PR doesn't relate to document changes
• The document should be updated later
• The document changes have been made in apache/echarts-doc#xxx

Misc

ZRender Changes

• This PR depends on ZRender changes (ecomfe/zrender#xxx).

Related test cases or examples to use the new APIs

N.A.

Others

Merging options

• Please squash the commits into a single one when merging.

Other information

[echarts-bot]

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⚠️ MISSING DOCUMENT INFO: Please make sure one of the document options are checked in this PR's description. Search "Document Info" in the description of this PR. This should be done either by the author or the reviewers of the PR.

[github-actions]

The changes brought by this PR can be previewed at: https://echarts.apache.org/examples/editor?version=PR-20865@294ed78