feat: add `isInitial` #6239

Job	Run time
SciMLBase.jl/Downstream/1	36m 18s
Catalyst.jl/All/1	34s
CellMLToolkit.jl/All/1	15m 31s
SBMLToolkit.jl/All/1	45m 24s
NeuralPDE.jl/NNPDE/1	23m 40s
DataDrivenDiffEq.jl/Downstream/1	14m 11s
StructuralIdentifiability.jl/All/1	27m 14s
ModelingToolkitStandardLibrary.jl/Core/1	33m 43s
ModelOrderReduction.jl/All/1	34s
MethodOfLines.jl/Interface/1	18m 59s
MethodOfLines.jl/2D_Diffusion/1	25m 12s
MethodOfLines.jl/DAE/1	20m 4s
ModelingToolkitNeuralNets.jl/All/1	43m 23s
	5h 4m 47s

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