fix(vrl): restore stdlib functions in CLI and playground

[pront]

Summary

• Restore the full VRL stdlib by default for the standalone VRL CLI and web playground.
• Enable that default directly with default = ["vrl/stdlib"] in both standalone crates.
• Preserve --no-default-features as the restricted build path for library consumers that intentionally avoid the full stdlib.

Root Cause

After the VRL feature split, the standalone VRL crates inherited the workspace vrl dependency with stdlib-base only. That unintentionally changed the default behavior for the standalone VRL CLI and web playground: functions from the full stdlib, such as get_env_var, were no longer available unless the build explicitly enabled vrl/stdlib.

Production Vector already enables vrl/stdlib through the root base feature. This change aligns the standalone CLI and web playground with that default production behavior, while consumers embedding Vector crates with default-features = false still do not enable the full stdlib unless they opt in.

Fixes #25267.

Validation

• Reproduced the get_env_var undefined-function error on v0.55.0 before the fix.
• cargo run -q -p vector-vrl-cli -- -q '.xxx = get_env_var("NODE_NAME") ?? "no_host"'
• cargo run -q -p vector-vrl-cli --no-default-features -- -q '.xxx = get_env_var("NODE_NAME") ?? "no_host"' still fails with E105
• cargo check -q -p vector-vrl-cli
• cargo check -q -p vector-vrl-web-playground
• cargo check -q -p vector-vrl-web-playground --no-default-features
• ./scripts/check_changelog_fragments.sh
• make check-clippy