[WebAssembly] Implement prototype f16x8.extract_lane instruction. (#93272)

Specified at:

https://github.com/WebAssembly/half-precision/blob/29a9b9462c9285d4ccc1a5dc39214ddfd1892658/proposals/half-precision/Overview.md

Note: the current spec has f16x8.extract_lane as opcode 0x124, but this
is incorrect and will be changed to 0x121 soon.