gh-39982: Implement conversion of log(2) to Giac
    
As in the title.

Tested locally and it works. I think the test won't be run on CI because
optional feature.

### 📝 Checklist

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- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [x] I have created tests covering the changes.
- [ ] I have updated the documentation and checked the documentation
preview.

--------

as far as I can tell the test failures are unrelated, but I've no idea
why they have to fail all at the same time. Maybe some download
corruption? (sounds quite unlikely…?)

#37066
    
URL: #39982
Reported by: user202729
Reviewer(s): Travis Scrimshaw

Author: Release Manager

src/sage/symbolic/constants.py

@@ -883,6 +883,8 @@ class Log2(Constant):
         0.69314718055994530941723212145817656807
         sage: RealField(150)(2).log()
         0.69314718055994530941723212145817656807550013
+        sage: giac(log2)  # optional - giac
+        ln(2)
     """
     def __init__(self, name='log2'):
         """
@@ -893,7 +895,7 @@ def __init__(self, name='log2'):
         """
         conversions = dict(mathematica='Log[2]', kash='Log(2)',
                            maple='log(2)', maxima='log(2)', gp='log(2)',
-                           pari='log(2)', octave='log(2)')
+                           pari='log(2)', octave='log(2)', giac='log(2)')
         Constant.__init__(self, name, conversions=conversions,
                           latex=r'\log(2)', domain='positive')