Fix window.__svrx__ undefined when using require.js

[xuchaoying]

Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests is needed?
  • Tests for the changes have been added
• Docs is needed?
  • Docs have been added / updated

What kind of change does this PR introduce?

• Bug fix
• Feature
• Enhencement
• Refactor
• Documents
• Others

1. Change webpack config libraryTarget of the injected svrx client script from umd to window, 'cause we only loaded through a <script> tag

What is the related issue?

#166

Does this PR introduce a breaking change?

• breaking change？

Other information:

[codecov]

Codecov Report

Merging #167 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #167   +/-   ##
======================================
  Coverage    95.7%   95.7%           
======================================
  Files          49      49           
  Lines        1675    1675           
======================================
  Hits         1603    1603           
  Misses         72      72

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