Handle magic insertion marks when merging external features #3194

Job	Run time
fontbe tests	48s
Clippy lints	1m 27s
Rustfmt	1m 9s
cargo check no std	1m 26s
fontc tests	1m 24s
tests other than fontbe,fontc	2m 9s
Build GS and check the result	3m 2s
resources/scripts/ots_test.sh	1m 11s
cargo check wasm	1m 10s
cargo test fea-rs	1m 3s
	14m 49s

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