Add 'r' flag (raw) for unquoted output

By default, placeholder expressions are automatically quoted to ensure
they are safely passed as arguments to external programs.

The r flag ({r}, {r1}, etc.) disables this behavior, outputting the
evaluated value without quotes.

For example,

  echo 'foo   bar' | fzf --preview 'echo {} {r}'

The preview command becomes:

  echo 'foo   bar' foo   bar

Since `{r}` expands to unquoted "foo   bar", 'foo' and 'bar' are passed
as separate arguments.

**Use with caution** Unquoted output can lead to broken commands.

  echo "let's go" | fzf --preview 'echo {r}'

Close #4330

src/terminal_test.go

@@ -75,6 +75,14 @@ func TestReplacePlaceholder(t *testing.T) {
 	result = replacePlaceholderTest("echo {}", true, Delimiter{}, printsep, false, "query", items1)
 	checkFormat("echo {{.O}}  foo{{.I}}bar baz{{.O}}")
 
+	// {r}, strip ansi
+	result = replacePlaceholderTest("echo {r}", true, Delimiter{}, printsep, false, "query", items1)
+	checkFormat("echo   foo'bar baz")
+
+	// {r..}, strip ansi
+	result = replacePlaceholderTest("echo {r..}", true, Delimiter{}, printsep, false, "query", items1)
+	checkFormat("echo foo'bar baz")
+
 	// {}, with multiple items
 	result = replacePlaceholderTest("echo {}", true, Delimiter{}, printsep, false, "query", items2)
 	checkFormat("echo {{.O}}foo{{.I}}bar baz{{.O}}")