Add zwasm to the standalone runtimes table - #563

Adds zwasm, a WebAssembly runtime written in Zig, to the Standalone Runtimes rows, plus its logo (images/zwasm.svg).

Every cell was measured against zwasm v2.5.0 (default build) rather than transcribed from project docs: the official spec-testsuite lanes run on both engines (interpreter and JIT), plus a minimal self-verifying probe module per feature. Where the engines differ, the cell carries a note naming which engine executes the feature and what the default mode does.

One question for the maintainers — non-blocking: zwasm ships two engines with an auto default that routes between them, so a few cells are capability-split rather than flag-gated: simd / relaxedSimd (the JIT executes them; the interpreter accepts the module but traps when a vector instruction executes), multiMemory (interpreter only; the JIT rejects), and threads (single-threaded host: wait times out, notify returns 0). Existing annotations in the table mostly describe how to enable a feature, though Wasmtime's tailCall note is a backend-and-architecture condition. Are these cells represented the way you would want, or would you rather see them as false, or worded some other way? If they are fine as written, nothing needs to change.

Versions are omitted deliberately (plain true) — per-feature "supported since" versions are not cheaply derivable from the project history.

Logo: silhouette derived from the WebAssembly logo mark (CC0-1.0, carlosbaraza/web-assembly-logo), recoloured, with lettering set in Open Sans (OFL-1.1).