Add WebP encoding support

Uses github.com/gen2brain/webp - a CGO-free library that uses WASM via
wazero runtime for encoding. WebP decoding was already supported.

- Add gen2brain/webp dependency for encoding
- Implement WebP encoding in processor.go
- Add FormatWebP to SupportedOutputFormats
- Re-enable WebP option in generator form dropdown
- Mark WebP encoding as complete in TODO.md

go.mod

@@ -52,10 +52,12 @@ require (
 	github.com/coreos/go-systemd/v22 v22.5.0 // indirect
 	github.com/davecgh/go-spew v1.1.2-0.20180830191138-d8f796af33cc // indirect
 	github.com/dustin/go-humanize v1.0.1 // indirect
+	github.com/ebitengine/purego v0.8.3 // indirect
 	github.com/emicklei/go-restful/v3 v3.12.1 // indirect
 	github.com/fatih/color v1.16.0 // indirect
 	github.com/felixge/httpsnoop v1.0.4 // indirect
 	github.com/fxamacker/cbor/v2 v2.9.0 // indirect
+	github.com/gen2brain/webp v0.5.5 // indirect
 	github.com/go-jose/go-jose/v4 v4.0.5 // indirect
 	github.com/go-logr/logr v1.4.2 // indirect
 	github.com/go-logr/stdr v1.2.2 // indirect
@@ -109,6 +111,7 @@ require (
 	github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec // indirect
 	github.com/ryanuber/go-glob v1.0.0 // indirect
 	github.com/spf13/pflag v1.0.10 // indirect
+	github.com/tetratelabs/wazero v1.9.0 // indirect
 	github.com/tidwall/gjson v1.18.0 // indirect
 	github.com/tidwall/match v1.1.1 // indirect
 	github.com/tidwall/pretty v1.2.0 // indirect