Add --offline for comp (#15623)

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_Please read the contribution guide: <https://doc.crates.io/contrib/>._

### What does this PR try to resolve?

Bash and Zsh `cargo add` completion option `--offline`

_Explain the motivation behind this change._
_A clear overview along with an in-depth explanation are helpful._

### How to test and review this PR?

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Author: ehuss

src/etc/_cargo

@@ -12,6 +12,7 @@ _cargo() {
         '(-q --quiet)*'{-v,--verbose}'[use verbose output]'
         '(-q --quiet -v --verbose)'{-q,--quiet}'[no output printed to stdout]'
         '-Z+[pass unstable (nightly-only) flags to cargo]: :_cargo_unstable_flags'
+        '--offline[run without accessing the network]'
         '--frozen[require that Cargo.lock and cache are up-to-date]'
         '--locked[require that Cargo.lock is up-to-date]'
         '--color=[specify colorization option]:coloring:(auto always never)'

src/etc/cargo.bashcomp.sh

@@ -49,7 +49,7 @@ _cargo()
 	local opt_targets="--lib --bin --bins --example --examples --test --tests --bench --benches --all-targets"
 
 	local opt___nocmd="$opt_common -V --version --list --explain"
-	local opt__add="$opt_common -p --package --features --default-features --no-default-features $opt_mani --optional --no-optional --rename --dry-run --path --git --branch --tag --rev --registry --dev --build --target --ignore-rust-version"
+	local opt__add="$opt_common -p --package --features --default-features --no-default-features $opt_mani $opt_lock --optional --no-optional --rename --dry-run --path --git --branch --tag --rev --registry --dev --build --target --ignore-rust-version"
 	local opt__bench="$opt_common $opt_pkg_spec $opt_feat $opt_mani $opt_lock $opt_jobs $opt_targets --message-format --target --no-run --no-fail-fast --target-dir --ignore-rust-version"
 	local opt__build="$opt_common $opt_pkg_spec $opt_feat $opt_mani $opt_lock $opt_parallel $opt_targets --message-format --target --release --profile --target-dir --ignore-rust-version"
 	local opt__b="$opt__build"