Implement support for @deprecated gates in wit-parser

[yoshuawuyts]

For the WASI 0.2 timeframe (early October), it would be great if we could land support for @deprecated gates (WebAssembly/component-model#377) in WIT, so we can begin closing out issues like WebAssembly/wasi-clocks#69 and WebAssembly/wasi-http#107.

The syntax we're proposing for this builds on the existing gate notation we have for @since and @unstable, extending it with a new @deprecated notation with a required version field and an optional message field:

gate ::= unstable-gate
       | since-gate
       | deprecated-gate

unstable-gate ::= '@unstable' '(' feature-field ')'
since-gate ::= '@since' '(' version-field ( ',' feature-field )? ')'
deprecated-gate ::= '@deprecated' '(' version-field ( ',' message-field )? ')'

feature-field ::= 'feature' '=' id
version-field ::= 'version' '=' <valid semver>
message-field ::= 'message' '=' <core:string>

I believe that #1508 probably did most of the heavy lifting required to implement gates, introducing the concept of extensible metadata to the parser. I'm hoping that adding this feature will be made a lot simpler because of that; perhaps as easy as extending the parser and adding a new entry in the metadata.

On the component model side we're again waiting for feedback from at least one implementation before landing the extension. It would be great if this project could again be that implementation. Thanks!