fix: handle untyped constant expressions in append() #2974

Job	Run time
build (1.20.x, GOARCH=amd64 GOOS=linux, gno)	49s
test (1.20.x, _test.cmd)	56s
build (1.20.x, GOARCH=wasm GOOS=js, gno)	32s
test (1.20.x, _test.pkg)	48s
build (1.21.x, GOARCH=amd64 GOOS=linux, gno)	35s
test (1.20.x, _test.gnolang.native)	26s
build (1.21.x, GOARCH=wasm GOOS=js, gno)	27s
test (1.20.x, _test.gnolang.stdlibs)	1m 6s
test (1.20.x, _test.gnolang.realm)	29s
test (1.20.x, _test.gnolang.pkg0)	2m 1s
test (1.20.x, _test.gnolang.pkg1)	6m 32s
test (1.20.x, _test.gnolang.pkg2)	4m 5s
test (1.20.x, _test.gnolang.other)	19s
test (1.21.x, _test.cmd)	58s
test (1.21.x, _test.pkg)	58s
test (1.21.x, _test.gnolang.native)	20s
test (1.21.x, _test.gnolang.stdlibs)	55s
test (1.21.x, _test.gnolang.realm)	31s
test (1.21.x, _test.gnolang.pkg0)	1m 36s
test (1.21.x, _test.gnolang.pkg1)	6m 5s
test (1.21.x, _test.gnolang.pkg2)	4m 8s
test (1.21.x, _test.gnolang.other)	28s
upload-coverage	14s
	35m 18s

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