feat(wasm): Add initial CMake build system and zstd-wasm target.

[junhaoliao]

Description

A prototype for the project is at https://github.com/junhaoliao/zstd-ffi-js/tree/prototype

1. Add deps setup tasks to download zstd sources and generate the single-file library as instructed at https://github.com/facebook/zstd/blob/22b44831637be02bff6a63f85e88671e4cb70460/build/single_file_libs/README.md
2. Add CMakeLists.txt which includes the zstd-wasm target.
3. Add tasks for building the target.

Checklist

• The PR satisfies the contribution guidelines.
• This is a breaking change and that has been indicated in the PR title, OR this isn't a
  breaking change.
• Necessary docs have been updated, OR no docs need to be updated.

Validation performed

1. Built the target with task or task zstd-wasm and observed no build error.

[hoophalab]

sorry just one more nitpick in the doc and the taskfile. xD

[hoophalab]

can we rename this as download-zstd since it only downloads tarball but doesn't create a cmake settings file. This keeps consistent with :deps:download-zstd in the doc.

(Or we can just rename :deps:download-zstd as :deps:install-zstd in the doc)