Improve interpolation design

[brandonchinn178]

Interpolatable is currently very small:

templette/templette/src/Templette/Prelude.hs

Lines 18 to 28 in 5fb1703

	-- TODO: move to another file?
	class Interpolatable a where
	interpolate :: a -> Text
	-- TODO: generalize? use similar thing as 'string-syntax'?
	instance Interpolatable Text where
	interpolate = id
	instance Interpolatable String where
	interpolate = Text.pack
	instance Interpolatable Int where
	interpolate = Text.pack . show

• We should probably add more instances (numerical instances, bool, etc.)
• Possibly generalize Interpolatable?
  • https://github.com/brandonchinn178/string-syntax/blob/6ffcf26f76f46769d42459c8f350d5bcc38a6ea6/src/Data/String/Syntax/Interpolate.hs#L68-L95
• Use interpolate in call directive?

  • templette/templette/src/Templette/Directives.hs

    Lines 36 to 37 in 5fb1703

    	-- TODO: Templette.Prelude.interpolate?
    	Right [TempletteOutputExpr $ func <> " (" <> renderInputContent content <> ")"]