operating point labels don't have "power" as autocomplete option

Description

When you use an operating point label for power (${OP:power} in a symbol field), autocomplete after ":" suggests the device's pin names, but not "power". "power" is a valid option here, so it would be more discoverable if "power" was suggested.

Steps to reproduce

1. Open attached project: op-power-autocomplete.zip
2. Double click the "power: " field on the right to edit it
3. Delete the text and enter ${OP:
4. Look at autocomplete options and see that "power" is not one of them

KiCad Version

Application: KiCad Schematic Editor arm64 on arm64

Version: 9.0.2-rc1-45-gcbd55bc201, release build

Libraries:
	wxWidgets 3.2.6
	FreeType 2.13.3
	HarfBuzz 10.2.0
	FontConfig 2.15.0

Platform: macOS Sonoma Version 14.7.2 (Build 23H311), 64 bit, Little endian, wxMac
OpenGL: Apple, Apple M1, 2.1 Metal - 88.1

	wxWidgets: 3.2.6 (wchar_t,wx containers)
	Boost: 1.87.0
	OCC: 7.8.1
	Curl: 8.7.1
	ngspice: 44.2
	Compiler: Clang 16.0.0 with C++ ABI 1002
	KICAD_IPC_API=ON