fix: handle untyped constant expressions in append() #2975

Job	Run time
build (1.20.x, GOARCH=amd64 GOOS=linux, gno)	20s
test (1.20.x, _test.cmd)	1m 12s
build (1.20.x, GOARCH=wasm GOOS=js, gno)	45s
test (1.20.x, _test.pkg)	39s
build (1.21.x, GOARCH=amd64 GOOS=linux, gno)	21s
build (1.21.x, GOARCH=wasm GOOS=js, gno)	31s
test (1.20.x, _test.gnolang.native)	32s
test (1.20.x, _test.gnolang.stdlibs)	52s
test (1.20.x, _test.gnolang.realm)	36s
test (1.20.x, _test.gnolang.pkg0)	1m 50s
test (1.20.x, _test.gnolang.pkg1)	6m 46s
test (1.20.x, _test.gnolang.pkg2)	4m 20s
test (1.20.x, _test.gnolang.other)	26s
test (1.21.x, _test.cmd)	1m 33s
test (1.21.x, _test.pkg)	41s
test (1.21.x, _test.gnolang.native)	23s
test (1.21.x, _test.gnolang.stdlibs)	44s
test (1.21.x, _test.gnolang.realm)	32s
test (1.21.x, _test.gnolang.pkg0)	1m 35s
test (1.21.x, _test.gnolang.pkg1)	6m 13s
test (1.21.x, _test.gnolang.pkg2)	4m 34s
test (1.21.x, _test.gnolang.other)	47s
upload-coverage	19s
	36m 31s

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