feat: allow customizing class prefix (#566)

[Justineo]

• 提供了 mixins/prefix，为组件提供了 $c 方法，可以根据编译时环境变量 VEUI_PREFIX 或 VUE_APP_VEUI_PREFIX（Vue CLI 需要用后者）决定组件输出的 class 前缀
• Less 中提供了 @veui-prefix 来支持自定义样式选择器的 class 前缀

[codecov]

Codecov Report

Merging #573 into theme-dls will increase coverage by <.01%.
The diff coverage is 64.28%.

@@              Coverage Diff              @@
##           theme-dls     #573      +/-   ##
=============================================
+ Coverage      75.39%   75.39%   +<.01%     
=============================================
  Files            152      154       +2     
  Lines           4909     4906       -3     
  Branches         996      994       -2     
=============================================
- Hits            3701     3699       -2     
+ Misses           848      847       -1     
  Partials         360      360

Impacted Files	Coverage Δ
packages/veui/src/components/Drawer.vue	83.33% <ø> (ø)	⬆️
packages/veui/src/components/Table/_ColGroup.js	100% <ø> (ø)	⬆️
packages/veui/src/components/Input.vue	67.64% <ø> (ø)	⬆️
packages/veui/src/components/Radio.vue	65% <ø> (ø)	⬆️
packages/veui/src/components/ButtonGroup.vue	40% <ø> (ø)	⬆️
packages/veui/src/components/Button.vue	92.3% <ø> (ø)	⬆️
packages/veui/src/components/Form/Fieldset.vue	100% <ø> (ø)	⬆️
packages/veui/src/components/Pagination.vue	68.57% <ø> (-0.45%)	⬇️
packages/veui/src/components/CheckButtonGroup.vue	70% <ø> (ø)	⬆️
packages/veui/src/components/Table/Table.vue	66.66% <ø> (ø)	⬆️
... and 65 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 978ae52...acceae8. Read the comment docs.