Add truecolor support for Windows, if available (#2156)

- Update to latest tcell which has 24 bit Windows support
- light renderer under Windows defaults to Dark256, if possible
- Respect TCELL_TRUECOLOR
- Remove tcell 1.3 references

go.mod

@@ -1,7 +1,7 @@
 module github.com/junegunn/fzf
 
 require (
-	github.com/gdamore/tcell v1.3.0
+	github.com/gdamore/tcell v1.4.0
 	github.com/lucasb-eyer/go-colorful v1.0.3 // indirect
 	github.com/mattn/go-isatty v0.0.12
 	github.com/mattn/go-runewidth v0.0.8