feat: add :map() method to Signal object

closes #19

Author: b0o

docs/pages/docs/signal.mdx

@@ -186,6 +186,69 @@ And here's the final result:
 
 <Method name="get_value" returns="table" />
 
+#### map
+
+> Applies a mapping function to each value emitted by the `Signal`. The mapping function receives the current value of the `Signal` object.
+
+<Method
+  name="map"
+  args={[
+    ['map_fn', 'fun(value: T): R'],
+  ]}
+  returns="SignalValue"
+/>
+
+Example:
+
+```lua
+local n = require 'nui-components'
+
+local renderer = n.create_renderer {
+  width = 60,
+  height = 12,
+}
+
+local signal = n.create_signal {
+  name = '',
+  description = '',
+}
+
+local body = function()
+  return n.rows(
+    n.paragraph {
+      is_focusable = false,
+      lines = signal:map(function(val)
+        return {
+          n.line(n.text('Name: ', 'Keyword'), val.name),
+          n.line(n.text('Description: ', 'Keyword'), val.description),
+        }
+      end),
+    },
+    n.gap(1),
+    n.form(
+      { id = 'form' },
+      n.text_input {
+        autofocus = true,
+        max_lines = 1,
+        border_label = 'Name',
+        on_change = function(val)
+          signal.name = val
+        end,
+      },
+      n.text_input {
+        max_lines = 1,
+        border_label = 'Description',
+        on_change = function(val)
+          signal.description = val
+        end,
+      }
+    )
+  )
+end
+
+renderer:render(body)
+```
+
 ### Methods / Operators (Signal Value)
 
 #### get_value

lua/nui-components/signal/init.lua

@@ -30,6 +30,12 @@ function Signal.create(object)
         end
       end
 
+      if key == "map" then
+        return function(_, ...)
+          return self:map(...)
+        end
+      end
+
       if key == "get_value" then
         return function(_)
           return self:get_value()
@@ -72,4 +78,8 @@ function Signal:get_value()
   return self._private.subject:get_value()
 end
 
+function Signal:map(map_fn)
+  return SignalValue.create(self._private.subject):map(map_fn)
+end
+
 return Signal

lua/nui-components/signal/value.lua

@@ -11,7 +11,7 @@ function SignalValue.create(subject, key)
       key = key,
       subject = subject,
       observable = subject:map(function(value)
-        return value[key]
+        return key ~= nil and value[key] or value
       end),
     },
   }
@@ -20,7 +20,8 @@ function SignalValue.create(subject, key)
 end
 
 function SignalValue:get_value()
-  return self._private.subject:get_value()[self._private.key]
+  local val = self._private.subject:get_value()
+  return self._private.key ~= nil and val[self._private.key] or val
 end
 
 function SignalValue:get_observer_value()