lsp: Use constant instead of repeating a `slint/showPreview` #17702

Job	Run time
cpp_test_driver (ubuntu-22.04)	6m 40s
build_and_test (ubuntu-22.04, 1.77)	12m 57s
cpp_test_driver (macos-13)	6m 56s
build_and_test (macos-13, stable)	34m 23s
cpp_test_driver (windows-2022)	8m 2s
ffi_32bit_build	1m 6s
build_and_test (windows-2022, stable)	15m 2s
cpp_cmake (macos-13, 1.77)	17m 26s
mcu (pico-st7789, thumbv6m-none-eabi)	30s
build_and_test (windows-2022, 1.77)	14m 52s
mcu_esp	2m 40s
cpp_cmake (windows-2022, nightly)	8m 4s
build_and_test (windows-2022, beta, --exclude ffmpeg --exclude gstreamer-player)	14m 52s
mcu (stm32h735g, thumbv7em-none-eabihf)	42s
cpp_cmake (ubuntu-22.04, stable)	6m 44s
build_and_test (ubuntu-22.04, nightly)	9m 54s
node_test (ubuntu-22.04)	2m 59s
node_test (macos-13)	4m 47s
python_test (ubuntu-22.04)	1m 33s
node_test (windows-2022)	7m 9s
vsce_build_test	5m 40s
python_test (macos-13)	1m 26s
python_test (windows-2022)	1m 50s
esp-idf-quick	9m 13s
android	1m 38s
fmt_test	1m 46s
miri	59s
updater_test (0.3.0)	1m 31s
docs / docs	6m 32s
wasm / wasm	3m 59s
format_lint_typecheck / format_lint_typecheck	2m 2s
tree-sitter / tree-sitter-tests	13s
wasm_demo / wasm_demo	2m 16s
wasm / build-slintpad	36s
cpp_package_test	2m 55s
	3h 39m 54s

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