[FEATURE]: Flexible tick label positioning on categorical axes (anchoring + between-category placement)

Description
Add support for more flexible positioning of tick labels on categorical axes, to correctly handle grouped or hierarchical categories—especially when groups contain an even number of items.
Specifically, this request proposes:

• Configurable horizontal text anchoring for tick labels (left, center, right), and
• Optional placement of tick labels at fractional (non‑data) axis positions, i.e. between two categorical ticks.

Currently, tick labels are always centre‑anchored on discrete tick positions, which prevents accurate or visually correct centring for even‑sized groups.

Why should this feature be added?
This is a recurring UX limitation in real‑world charts that use:

• grouped or hierarchical categorical axes,
• experimental or test‑matrix layouts,
• manufacturing and operational visualisations.

For even‑sized groups, the true visual centre lies between two categories, but:

• tick labels cannot be placed between ticks,
• text anchoring cannot be adjusted,
• whitespace or HTML hacks are neutralised by automatic centring,
• annotations are not a clean substitute for tick labels.

Providing:

• anchor control (left / centre / right) would address many alignment issues with minimal complexity, while
• fractional tick‑label positioning would solve the problem exactly for grouped categories.

Together, these features would significantly improve readability, reduce workarounds, and enable precise hierarchical labelling without breaking Plotly’s axis model.

Mocks / Designs
Conceptually:

• a grouped label whose left edge aligns with a group boundary (e.g. at x = 1.5),
• or a label centred between two categorical ticks rather than on a data point.

A simple screenshot of a grouped categorical axis with an even number of categories clearly illustrates the current limitation.

Notes

• Applicable to plotly.js and all language bindings (Python, R, etc.).
• Most visible when compressing or centring hierarchical tick labels.
• Proposed as:
  • Primary feature: tick‑label horizontal anchoring
  • Advanced extension: fractional / between‑category tick placement

该 issue 未指定文件、测试或入口点；首先在 plotly.js 中定位分类轴刻度标签的渲染代码及其现有测试。明确水平锚定与分数定位之间的范围，然后验证分组的分类标签可以左对齐、居中或右对齐，并验证类别之间的位置表现正确。