fix: add main package for ergocheck to enable go install

Fixes #3

The ergocheck package was a library package, not a main package,
so `go install github.com/newmo-oss/ergo/ergocheck@v0.1.0` failed.

Added cmd/ergocheck/main.go with singlechecker.Main() to make it
installable as a CLI tool.

Signed-off-by: sivchari <shibuuuu5@gmail.com>

Author: sivchari

ergocheck/README.ja.md

@@ -65,7 +65,7 @@ var ErrNotFound = ergo.NewSentinel("not found")
 ## インストール
 
 ```bash
-go install github.com/newmo-oss/ergo/ergocheck@latest
+go install github.com/newmo-oss/ergo/ergocheck/cmd/ergocheck@latest
 ```
 
 ## 使い方

ergocheck/README.md

@@ -65,7 +65,7 @@ var ErrNotFound = ergo.NewSentinel("not found")
 ## Installation
 
 ```bash
-go install github.com/newmo-oss/ergo/ergocheck@latest
+go install github.com/newmo-oss/ergo/ergocheck/cmd/ergocheck@latest
 ```
 
 ## Usage

ergocheck/cmd/ergocheck/main.go

@@ -0,0 +1,10 @@
+package main
+
+import (
+	"github.com/newmo-oss/ergo/ergocheck"
+	"golang.org/x/tools/go/analysis/singlechecker"
+)
+
+func main() {
+	singlechecker.Main(ergocheck.Analyzer)
+}