Add codecov reporting

[knolleary]

Description

This modifies our builds and npm actions to run code coverage reporting.

• Main build action:
  • Removes the frontend unit tests as they are also run in the e2e action - we don't want to be reporting on them twice
  • Adds upload to codecov with the 'backend' tag applied
• E2E build - updated
  • Adds upload to codecov with the 'frontend' tag applide
• Updates vitest and adds the istanbul coverage reporter - needed for compatibility with the nyc reports.
• Updates the cover:* actions to generating their coverage reports in a consistent location and renames them so they are individually identifiable. (nyc lets you pick the dir it'll write the json to, but no the name of the json file - so we have to do this dance to get the files from separate runs into the same dir without overwriting each other)

[knolleary]

Merging this PR will get a baseline of coverage stats into codecov. From then on, subsequent PRs should get a coverage report added.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (main@183abd5). Click here to learn what that means.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #2437   +/-   ##
=======================================
  Coverage        ?   38.87%           
=======================================
  Files           ?      489           
  Lines           ?    17078           
  Branches        ?     3955           
=======================================
  Hits            ?     6639           
  Misses          ?    10439           
  Partials        ?        0           

Flag	Coverage Δ
backend	72.32% <0.00%> (?)
frontend	1.55% <0.00%> (?)

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