Normalize WAT codeblock syntax names

[mkatychev]

The repo seems to have a mix of ```wat and ```wasm codeblocks:

component-model/design/mvp/WIT.md

Lines 114 to 121 in 53ee310

	```wasm
	(component
	(import "local:demo/host" (instance $host
	(export "log" (func (param "msg" string)))
	))
	;; ...
	)
	```

component-model/design/mvp/WIT.md

Lines 1985 to 1993 in 53ee310

	```wat
	(component
	(type (export "i") (component
	(export "ns:p/i@1.0.0" (instance
	(export "f" (func))
	))
	))
	)
	```

Though the ```wasm syntax declaration does seem to syntax the WAT s-expressions correctly, the mixed use led to a bit of confusion on my part.

After a bit of reading, WAT is defined as the "retranscription of the [WASM] binary format in text".

This PR replaces codeblocks declared as "wasm" to "wat".

[mkatychev]

I may be entirely wrong in this conclusion so pushback/feedback is welcome.

[lukewagner]

wat sounds right to me; thanks!