fix: enable strongly-typed enum SiliconChargeSharingConfig::ESigmaMode

[wdconinc]

Briefly, what does this PR introduce?

This PR modifies the unscoped enum ESigmaMode to a scoped enum, and avoids the implicit integer conversion by correcting the operator<< signature. Before this fix, the output was 0 or 1 (since the incorrectly-signatured operator<< is not used); after this fix, the output is abs or rel.

The scoping of the enum adds a layer of safety since the symbol abs could previously shadow the standard function std::abs.

This aligns with the enabled clang-tidy check cppcoreguidelines-use-enum-class, https://clang.llvm.org/extra/clang-tidy/checks/cppcoreguidelines/use-enum-class.html.

What kind of change does this PR introduce?

• Bug fix (issue: fix: Add ImagingTopoCluster layer mode within same layers #1999 (comment), but fails to compile fix: Add ImagingTopoCluster layer mode within same layers #1999 (comment))
• New feature (issue #__)
• Documentation update
• Other: __

Please check if this PR fulfills the following:

• Tests for the changes have been added
• Documentation has been added / updated
• Changes have been communicated to collaborators

Does this PR introduce breaking changes? What changes might users need to make to their code?

No.

Does this PR change default behavior?

No.

[wdconinc]

Rebasing for #2057.