Add len function for String #23414

Job	Run time
build-forc-doc-sway-lib-std	1m 29s
build-mdbook	2m 16s
build-reference-examples	1m 5s
build-forc-test-project	2m 2s
build-sway-examples	58s
build-sway-lib-core	53s
build-sway-lib-std	1m 2s
cargo-build-workspace	2m 10s
cargo-clippy	39s
cargo-fmt-check	11s
cargo-run-e2e-test-evm	2m 17s
cargo-test-forc	1m 30s
cargo-test-lib-std	56s
cargo-test-sway-lsp	2m 11s
cargo-test-workspace	2m 15s
cargo-toml-fmt-check	13s
cargo-unused-deps-check	2m 11s
check-sdk-harness-test-suite-compatibility	25s
forc-fmt-check-panic	2m 6s
forc-fmt-check-sway-examples	42s
forc-fmt-check-sway-lib-core	46s
forc-fmt-check-sway-lib-std	56s
forc-pkg-fuels-deps-check	13s
forc-run-benchmarks	2m 4s
forc-unit-tests	1m 40s
get-fuel-core-version	24s
pre-publish-check	0s
cargo-run-e2e-test	1m 21s
cargo-run-e2e-test-release	1m 18s
cargo-test-forc-debug	1m 18s
cargo-test-forc-client	1m 21s
cargo-test-forc-node	1m 4s
build-publish-master-image	1s
notify-slack-on-failure	1s
publish	1s
verifications-complete	1s
build-publish-release-image	1s
Build and upload forc binaries to release	1s
	40m 2s

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