chore(plugin-webpack): use trailing slash in dev mode entrypoint baseUrl

[dsanders11]

• [x] I have read the contribution documentation for this project.
• [x] I agree to follow the code of conduct that this project follows, as appropriate.
• [ ] The changes are appropriately documented (if applicable).
• [ ] The changes have sufficient test coverage (if applicable).
• [ ] The testsuite passes successfully on my local machine (if applicable).

Summarize your changes:

This better reflects the reality that it's a directory, and it makes it easier to construct relative URLs via new URL('foo/', window.location.href) which would otherwise strip the entrypoint name.

[dsanders11]

This seems more like a fix: than a chore: to me, but what's your reasoning?

Yea, I went back and forth on which to label it. It's not technically a bug, it's more of an usability improvement. Happy to change it to fix: if you'd prefer that.

Is there a reasonable way to add regression tests for this?

I think so, looks like it's failing this test anyway: "sets the renderer entry point to an HTML file if both an HTML & JS file are specified:" due to the extract slash. I'll write some additional test coverage. Looks like it also broke a couple of tests. I'm guilty of not testing, I fired this off during a larger debugging session on the Fiddle GSoC stuff.

Will move this to draft for the moment and fix it up during the week.

[codecov[bot]]

Codecov Report

Merging #2890 (caef034) into master (067a68c) will decrease coverage by 14.85%. The diff coverage is n/a.

@@             Coverage Diff             @@
##           master    #2890       +/-   ##
===========================================
- Coverage   71.40%   56.55%   -14.86%     
===========================================
  Files          79       48       -31     
  Lines        2385     1457      -928     
  Branches      449      295      -154     
===========================================
- Hits         1703      824      -879     
- Misses        454      530       +76     
+ Partials      228      103      -125     

Impacted Files	Coverage Δ
packages/api/core/src/util/upgrade-forge-config.ts	10.52% <0.00%> (-87.72%)	:arrow_down:
packages/api/core/src/util/publish-state.ts	12.82% <0.00%> (-79.49%)	:arrow_down:
packages/api/core/src/util/electron-executable.ts	24.00% <0.00%> (-76.00%)	:arrow_down:
packages/maker/snap/src/MakerSnap.ts	40.00% <0.00%> (-60.00%)	:arrow_down:
packages/api/core/src/api/start.ts	15.66% <0.00%> (-50.22%)	:arrow_down:
packages/maker/wix/src/util/author-name.ts	50.00% <0.00%> (-50.00%)	:arrow_down:
packages/maker/appx/src/util/author-name.ts	50.00% <0.00%> (-50.00%)	:arrow_down:
packages/api/core/src/api/publish.ts	18.42% <0.00%> (-50.00%)	:arrow_down:
packages/maker/flatpak/src/MakerFlatpak.ts	33.33% <0.00%> (-43.94%)	:arrow_down:
packages/api/core/src/util/electron-version.ts	32.30% <0.00%> (-43.08%)	:arrow_down:
... and 58 more

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