Fix: Additional layers in stackup not parsed

[mvnmgrx]

This PR fixes #17 and implements sublayer tokens to parse dielectric stackups in a PCB correctly.

• Implemented class StackupSubLayer
• Added test case for stacked dielectrics

Tests:

✅ - 0.002s - Testing <class 'src.kiutils.libraries.LibTable'> to compare for case fp-lib-table. result: Success
✅ - 0.001s - Testing <class 'src.kiutils.libraries.LibTable'> to compare for case sym-lib-table. result: Success
✅ - 0.012s - Testing <class 'src.kiutils.schematic.Schematic'> to equality for case test.kicad_sch. result: Success
✅ - 0.023s - Testing <class 'src.kiutils.board.Board'> to equality for case test.kicad_pcb. result: Success
✅ - 0.030s - Testing <class 'src.kiutils.footprint.Footprint'> to compare for case test.kicad_mod. result: Success
✅ - 0.049s - Testing <class 'src.kiutils.symbol.SymbolLib'> to compare for case test.kicad_sym. result: Success
✅ - 0.006s - Testing <class 'src.kiutils.footprint.Footprint'> to equality for case test_fp_all.kicad_mod. result: Success
✅ - 0.002s - Testing <class 'src.kiutils.footprint.Footprint'> to compare for case test_fp_empty_attr.kicad_mod. result: Success
✅ - 0.002s - Testing <class 'src.kiutils.symbol.SymbolLib'> to compare for case test_sym_demorgan.kicad_sym. result: Success
✅ - 0.009s - Testing <class 'src.kiutils.symbol.SymbolLib'> to compare for case test_sym_demorgan_syitems.kicad_sym. result: Success
✅ - 0.002s - Testing <class 'src.kiutils.symbol.SymbolLib'> to compare for case test_sym_parameters.kicad_sym. result: Success
✅ - 0.011s - Testing <class 'src.kiutils.symbol.SymbolLib'> to compare for case test_sym_pins.kicad_sym. result: Success
✅ - 0.006s - Testing <class 'src.kiutils.symbol.SymbolLib'> to compare for case test_sym_alternate_pins.kicad_sym. result: Success
✅ - 0.035s - Testing <class 'src.kiutils.wks.WorkSheet'> to compare for case test_wks_all.kicad_wks. result: Success
✅ - 0.002s - Testing <class 'src.kiutils.dru.DesignRules'> to compare for case test_dru_all.kicad_dru. result: Success
✅ - 0.007s - Testing <class 'src.kiutils.footprint.Footprint'> to compare for case test_fp_pad_newlines.kicad_mod. result: Success
✅ - 0.019s - Testing <class 'src.kiutils.board.Board'> to compare for case test_pcb_trace_arcs.kicad_pcb. result: Success
✅ - 0.010s - Testing <class 'src.kiutils.board.Board'> to compare for case test_pcb_stackup_dielectrics_32layer_vias.kicad_pcb. result: Success

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✅ KiUtils tests done, all green ✅