feat(comp: backtop): add backtop component (#79)

[imguolao]

fix #79

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• The commit message follows our guidelines
• Tests for the changes have been added/updated or not needed
• Docs and demo have been added/updated or not needed

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Component style update
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Application (the showcase website) / infrastructure changes
• Other... Please describe:

What is the current behavior?

What is the new behavior?

Other information

[imguolao]

@danranVm @huangtenghui need review.

[codecov]

Codecov Report

Merging #185 (deabe66) into main (b6d178c) will increase coverage by 0.14%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #185      +/-   ##
==========================================
+ Coverage   95.88%   96.03%   +0.14%     
==========================================
  Files          74       76       +2     
  Lines        1557     1614      +57     
  Branches      265      275      +10     
==========================================
+ Hits         1493     1550      +57     
  Misses         33       33              
  Partials       31       31              

Impacted Files	Coverage Δ
packages/components/icon/src/icons.ts	100.00% <ø> (ø)
packages/components/back-top/src/BackTop.vue	100.00% <100.00%> (ø)
packages/components/core/config/defaultConfig.ts	100.00% <100.00%> (ø)
...es/components/icon/definitions/verticalAlignTop.ts	100.00% <0.00%> (ø)

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[danranVm]

LGTM.