feat(accordion): use readonly arrays for inputs (#2773)

[reduckted]

Before submitting a pull request, please make sure you have at least performed the following:

• read and followed the CONTRIBUTING.md guide.
• built and tested the changes locally.
• added/updated any applicable tests.
• added/updated any applicable API documentation.
• added/updated any applicable demos.

Part of #2773.

[maxokorokov]

Left a comment, thanks!

[codecov-io]

Codecov Report

Merging #3426 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #3426   +/-   ##
=======================================
  Coverage   91.13%   91.13%           
=======================================
  Files          96       96           
  Lines        2797     2797           
  Branches      516      516           
=======================================
  Hits         2549     2549           
  Misses        190      190           
  Partials       58       58

Flag	Coverage Δ
#e2e	56.16% <100%> (ø)	⬆️
#unit	87.97% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
src/accordion/accordion.ts	98.43% <100%> (ø)	⬆️

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[maxokorokov]

LGTM, thanks!

Sorry for the delay.