feat(installer): use go-git to clone

Integrate `go-git` into the application, and use it to `git clone`
cheatsheets when the installer runs.

Previously, the installer required that `git` be installed on the system
`PATH`, so this change has to big advantages:

1. It removes that system dependency on `git`
2. It paves the way for implementing the `--update` command

Additionally, `cheat` now performs a `--depth=1` clone when installing
cheatsheets, which should at least somewhat improve installation times
(especially on slow network connections).

vendor/github.com/go-git/go-billy/v5/osfs/os_js.go

@@ -0,0 +1,21 @@
+// +build js
+
+package osfs
+
+import (
+	"github.com/go-git/go-billy/v5"
+	"github.com/go-git/go-billy/v5/helper/chroot"
+	"github.com/go-git/go-billy/v5/memfs"
+)
+
+// globalMemFs is the global memory fs
+var globalMemFs = memfs.New()
+
+// Default Filesystem representing the root of in-memory filesystem for a
+// js/wasm environment.
+var Default = memfs.New()
+
+// New returns a new OS filesystem.
+func New(baseDir string) billy.Filesystem {
+	return chroot.New(Default, Default.Join("/", baseDir))
+}