[Strings2] Improve string repr of Sum

[Qottmann]

Sum instances are now displayed multiline and without unnecessary brackets. Further, all outputs should be copy-and-paste-able. Some examples how this now looks:

>>> qml.operation.enable_new_opmath()
>>> X(0) + X(1) + X(2) # short outputs <60 characters in single line
X(0) + X(1) + X(2)
>>> 0.5 * (X(0) @ X(1)) + 0.7 * (X(1) @ X(2)) + 0.8 * (X(2) @ X(3)) # larger outputs break into multiple lines. copy-paste-able with surrounding brackets
(
    0.5 * (X(0) @ X(1))
  + 0.7 * (X(1) @ X(2))
  + 0.8 * (X(2) @ X(3))
)

There are also some peculiarities that cannot be improved at the moment because they are due to the nesting and lazy execution of products and sums:

>>> 0.5 * (X(0) @ (0.5 * X(1))) + 0.7 * X(1)
0.5 * (X(0) @ (0.5 * X(1))) + 0.7 * X(1)

ToDo

• Basic functionality
• Tests
• multiline threshold solution? (Problem from nesting in e.g. X(0) + X(1) + X(2))
• changelog

Documentation will be updated together with the changes from #5116 in a follow-up PR

Builds on top of #5116
[sc-54959]
[sc-54955]

[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit doc/releases/changelog-dev.md with:

• A one-to-two sentence description of the change. You may include a small working example for new features.
• A link back to this PR.
• Your name (or GitHub username) in the contributors section.

[trbromley]

Thanks!

[albi3ro]

🎉 🚀 Thank you so much!

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (0428922) 99.68% compared to head (15fa31b) 99.67%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5138      +/-   ##
==========================================
- Coverage   99.68%   99.67%   -0.01%     
==========================================
  Files         394      394              
  Lines       36298    36034     -264     
==========================================
- Hits        36183    35918     -265     
- Misses        115      116       +1     

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[albi3ro]