Add Astronoby::Moon#current_phase_fraction (#115)

While `#illuminated_fraction` already provides a valid information on how much the Moon seems illuminated as observed from the Earth, it doesn't help deduce the current phase name. When the Moon is 45% illuminated, it can be either the First Quarter or the Last Quarter, there is no way of knowing without calculating when is was the previous Full Moon and when is the next one.

To help with this, we introduce `#current_phase_fraction`, based on the mean elongation, which is a number from 0 to 1 which always increase while we're getting closer to the next New Moon.

As it is using `#mean_elongation`, the result is not extremely precise, however it is precise enough to deduce the current phase name for a given day.

For a user wanting to get accurate phase times, it is preferred to use `::monthly_phase_events`.

```rb
# ~ time of first quarter in December 2024
time = Time.utc(2024, 12, 8, 15, 26)

Astronoby::Moon.new(time: time)
  .illuminated_fraction
  .round(2)
# => 0.5

Astronoby::Moon.new(time: time)
  .current_phase_fraction
  .round(2)
# => 0.26

# ~ time of last quarter in December 2024
time = Time.utc(2024, 12, 22, 22, 18)

Astronoby::Moon.new(time: time)
  .illuminated_fraction
  .round(2)
# => 0.5

Astronoby::Moon.new(time: time)
  .current_phase_fraction
  . round(2)
# => 0.74
```

Fixes #101

README.md

@@ -249,6 +249,9 @@ moon = Astronoby::Moon.new(time: time)
 moon.illuminated_fraction.round(2)
 # => 0.31
 
+moon.current_phase_fraction.round(2)
+# => 0.82
+
 moon.distance.km.round
 # => 368409
 

lib/astronoby/bodies/moon.rb

@@ -167,6 +167,11 @@ def illuminated_fraction
       @illuminated_fraction ||= (1 + phase_angle.cos) / 2
     end
 
+    # @return [Float] Phase fraction, from 0 to 1
+    def current_phase_fraction
+      mean_elongation.degrees / Constants::DEGREES_PER_CIRCLE
+    end
+
     # @param observer [Astronoby::Observer] Observer of the event
     # @return [Astronoby::Events::ObservationEvents] Moon's observation events
     def observation_events(observer:)

spec/astronoby/bodies/moon_spec.rb

@@ -398,6 +398,26 @@
     end
   end
 
+  describe "#current_phase_fraction" do
+    it "returns the mean elongation's fraction" do
+      moon = described_class.new(time: Time.new)
+      allow(moon).to receive(:mean_elongation)
+        .and_return(Astronoby::Angle.from_degrees(90))
+
+      phase_fraction = moon.current_phase_fraction
+
+      expect(phase_fraction).to eq 0.25
+    end
+
+    it "returns the mean elongation's fraction for 2024-01-01" do
+      moon = described_class.new(time: Time.utc(2024, 1, 1))
+
+      phase_fraction = moon.current_phase_fraction
+
+      expect(phase_fraction.round(2)).to eq 0.66
+    end
+  end
+
   describe "#observation_events" do
     describe "#rising_time" do
       it "returns the moonrise time on 1991-03-14" do