Launch Preview 2

[pchickey]

Pending vote in 1/25 meeting, this changes preview2/README.md to reflect the launched status of the standard, and adds the 0.2.0 wits for each proposal.

See also:

• https://github.com/WebAssembly/wasi-io/pull/68
• https://github.com/WebAssembly/wasi-random/pull/40
• https://github.com/WebAssembly/wasi-clocks/pull/60
• https://github.com/WebAssembly/wasi-filesystem/pull/144
• https://github.com/WebAssembly/wasi-sockets/pull/99
• https://github.com/WebAssembly/wasi-cli/pull/41
• https://github.com/WebAssembly/wasi-http/pull/98

[pchickey]

The votes to include wasi:cli/command and wasi:http/proxy worlds in Preview 2, and then to launch preview 2, have passed in the 1/25 WASI Subgroup meeting. I'll let Dan do the honors here.