examples(pug): fix example by adding plain-pug-loader

[paulgeisler]

Add pug dependencies used by vue-loader v15 to fix the related example.

Types of changes

• Bug fix (a non-breaking change which fixes an issue)

Resolves #5212

[codecov-io]

Codecov Report

Merging #5223 into dev will increase coverage by 44.42%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##              dev    #5223       +/-   ##
===========================================
+ Coverage   51.87%   96.29%   +44.42%     
===========================================
  Files          73       73               
  Lines        2483     2483               
  Branches      631      631               
===========================================
+ Hits         1288     2391     +1103     
+ Misses        926       77      -849     
+ Partials      269       15      -254

Impacted Files	Coverage Δ
packages/webpack/src/config/server.js	100% <0%> (+4.16%)	⬆️
packages/webpack/src/utils/perf-loader.js	100% <0%> (+5.26%)	⬆️
packages/webpack/src/config/base.js	95.45% <0%> (+5.68%)	⬆️
packages/utils/src/lang.js	100% <0%> (+6.25%)	⬆️
packages/core/src/module.js	98.33% <0%> (+10%)	⬆️
packages/utils/src/context.js	100% <0%> (+11.11%)	⬆️
packages/cli/src/commands/index.js	11.11% <0%> (+11.11%)	⬆️
packages/webpack/src/config/client.js	96.29% <0%> (+12.96%)	⬆️
packages/utils/src/resolve.js	96.07% <0%> (+13.72%)	⬆️
packages/config/src/config/build.js	100% <0%> (+14.28%)	⬆️
... and 38 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 ff93d14...347da75. Read the comment docs.