Extend OptimKit.fixedpoint instead of exporting a separate fixedpoint

[leburgel]

OptimKit 0.5 exports fixedpoint. Because PEPSKit exports a fixedpoint of its own, an unqualified fixedpoint after using PEPSKit, OptimKit throws an UndefVarError, which breaks the example tests and the code shown in the docs. We now add PEPSKit's methods to OptimKit.fixedpoint and require OptimKit 0.5.

Conceptually the two functions have little to do with each other, so this is simply the easiest solution. We could also keep PEPSKit's fixedpoint separate, but then every script that uses both PEPSKit and OptimKit breaks.

Reproducer

using PEPSKit, OptimKit
fixedpoint

🤖 Generated with Claude Code

[lkdvos]

Hmmm, this is not the prettiest but I guess it will do 🤷

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
src/PEPSKit.jl	100.00% <ø> (ø)

... and 10 files with indirect coverage changes

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