Wrap reflect.Type.ConvertibleTo calls with Go/TinyGo implementations

Enable package to be usable with [TinyGo](https://tinygo.org).

Changes calls to reflect.Type.ConvertibleTo in decode.go to Go/TinyGo
specific implementations. Basic examples work, but (some) unit tests
fail as conversions to `interface{}` do not currently work in TinyGo.

Otherwise, all other functionality I've tested now appears to work
properly: highlighted error output, `cmd/ycat`, the README.md examples,
etc.

As an example, both Go and TinyGo are now able to use this package
vis-a-vis this example taken from README.md:

```go
package main

import (
	"fmt"
	"strings"

	"github.com/goccy/go-yaml"
)

func main() {
	yml := `
store:
  book:
    - author: john
      price: 10
    - author: ken
      price: 12
  bicycle:
    color: red
    price: 19.95
`
	path, err := yaml.PathString("$.store.book[*].author")
	if err != nil {
		//...
	}
	var authors []string
	if err := path.Read(strings.NewReader(yml), &authors); err != nil {
		//...
	}
	fmt.Println(authors)
}
```

Output:

```sh
ken@ken-desktop:~/g$ go run main.go
[john ken]
ken@ken-desktop:~/g$ tinygo run main.go
[john ken]
ken@ken-desktop:~/g$
```

(previously the `tinygo run` would panic)

conv_go.go

@@ -0,0 +1,11 @@
+//go:build !tinygo
+
+package yaml
+
+import (
+	"reflect"
+)
+
+func convertibleTo(src reflect.Value, typ reflect.Type) bool {
+	return src.Type().ConvertibleTo(typ)
+}

conv_tinygo.go

@@ -0,0 +1,65 @@
+//go:build tinygo
+
+package yaml
+
+import (
+	"reflect"
+)
+
+func convertibleTo(src reflect.Value, typ reflect.Type) bool {
+	srck, typk := src.Kind(), typ.Kind()
+	if srck == typk {
+		return true
+	}
+	switch srck {
+	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
+		switch typk {
+		case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64, reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
+			return true
+		case reflect.Float32, reflect.Float64:
+			return true
+		case reflect.String:
+			return true
+		}
+
+	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
+		switch typk {
+		case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64, reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
+			return true
+		case reflect.Float32, reflect.Float64:
+			return true
+		case reflect.String:
+			return true
+		}
+
+	case reflect.Float32, reflect.Float64:
+		switch typk {
+		case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
+			return true
+		case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
+			return true
+		case reflect.Float32, reflect.Float64:
+			return true
+		}
+
+	case reflect.Slice:
+		if typk == reflect.String /*&& !src.Type().Elem().isNamed()*/ {
+			switch src.Type().Elem().Kind() {
+			case reflect.Uint8, reflect.Int32:
+				return true
+			}
+		}
+
+	case reflect.String:
+		switch typk {
+		case reflect.Slice:
+			switch typ.Elem().Kind() {
+			case reflect.Uint8, reflect.Int32:
+				return true
+			}
+			return false
+		}
+	}
+
+	return false
+}

decode.go

@@ -562,7 +562,7 @@ func (d *Decoder) getArrayNode(node ast.Node) (ast.ArrayNode, error) {
 
 func (d *Decoder) convertValue(v reflect.Value, typ reflect.Type, src ast.Node) (reflect.Value, error) {
 	if typ.Kind() != reflect.String {
-		if !v.Type().ConvertibleTo(typ) {
+		if !convertibleTo(v, typ) {
 
 			// Special case for "strings -> floats" aka scientific notation
 			// If the destination type is a float and the source type is a string, check if we can
@@ -593,7 +593,7 @@ func (d *Decoder) convertValue(v reflect.Value, typ reflect.Type, src ast.Node)
 	case reflect.Bool:
 		return reflect.ValueOf(fmt.Sprint(v.Bool())), nil
 	}
-	if !v.Type().ConvertibleTo(typ) {
+	if !convertibleTo(v, typ) {
 		return reflect.Zero(typ), errTypeMismatch(typ, v.Type(), src.GetToken())
 	}
 	return v.Convert(typ), nil
@@ -1646,7 +1646,7 @@ func (d *Decoder) decodeMap(ctx context.Context, dst reflect.Value, src ast.Node
 				return err
 			}
 			k = reflect.ValueOf(keyVal)
-			if k.IsValid() && k.Type().ConvertibleTo(keyType) {
+			if k.IsValid() && convertibleTo(k, keyType) {
 				k = k.Convert(keyType)
 			}
 		}