feat: create separate package to make reactor modules easier

example/reactor/README.md

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+## Reactor module example
+By including this package, you'll turn your plugin into a [Reactor](https://dylibso.com/blog/wasi-command-reactor/) module. This makes sure that you can use WASI (e.g. File Access) in your exported functions.
+
+To test this example, run:
+
+```
+tinygo build -target wasi -o reactor.wasm .\tiny_main.go
+extism call ./reactor.wasm read_file --input "./test.txt" --allow-path . --wasi --log-level info
+// => Hello World!
+```
+
+If you don't include the pacakge, you'll see this output:
+```
+extism call .\c.wasm read_file --input "./test.txt" --allow-path . --wasi --log-level info
+// => 2024/01/18 20:48:48 open ./test.txt: errno 76
+```

example/reactor/test.txt

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+Hello World!

example/reactor/tiny_main.go

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+//go:build !std
+// +build !std
+
+package main
+
+import (
+	"os"
+
+	"github.com/extism/go-pdk"
+	_ "github.com/extism/go-pdk/pdk_reactor"
+)
+
+//export read_file
+func read_file() {
+	name := pdk.InputString()
+
+	content, err := os.ReadFile(name)
+	if err != nil {
+		pdk.Log(pdk.LogError, err.Error())
+		return
+	}
+
+	pdk.Output(content)
+}
+
+func main() {}

go.work

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+go 1.21.1
+
+use (
+	.
+	./pdk_reactor
+)

pdk_reactor/extism_pdk_reactor.go

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+package pdk_reactor
+
+//export __wasm_call_ctors
+func __wasm_call_ctors()
+
+//export _initialize
+func _initialize() {
+	__wasm_call_ctors()
+}

pdk_reactor/go.mod

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+module github.com/extism/go-pdk/pdk_reactor
+
+go 1.21.1