Fix: use UMD externals

[mwistrand]

Since AMD externals cannot be used with UMD modules, update the
externals config setting to use a UMD object instead of a string that
declares the external's module type.

[codecov]

Codecov Report

Merging #133 into master will increase coverage by 0.12%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #133      +/-   ##
==========================================
+ Coverage   57.42%   57.55%   +0.12%     
==========================================
  Files           8        8              
  Lines         451      450       -1     
  Branches      101      101              
==========================================
  Hits          259      259              
+ Misses        192      191       -1

Impacted Files	Coverage Δ
src/base.config.ts	25.89% <0%> (+0.22%)	⬆️

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