Base.get_extension & Dates.format made public (#58108)

`Base.get_extension` and `Dates.format` both appear in the manual and
should therefore be `public` symbols according to
[51335](#51335 (comment)).

They appear in the manual
[here](https://docs.julialang.org/en/v1/base/base/#Base.get_extension)
and

[here](https://docs.julialang.org/en/v1/stdlib/Dates/#Dates.format-Tuple%7BTimeType,%20AbstractString%7D).

Please also consider back porting this to version 1.12

(cherry picked from commit 963eaa7)

Author: GHTaarn

base/exports.jl

@@ -1121,6 +1121,7 @@ public
     DL_LOAD_PATH,
     load_path,
     active_project,
+    get_extension,
 
 # Reflection and introspection
     isambiguous,

stdlib/Dates/src/Dates.jl

@@ -81,4 +81,6 @@ export Period, DatePeriod, TimePeriod,
        # io.jl
        ISODateTimeFormat, ISODateFormat, ISOTimeFormat, DateFormat, RFC1123Format, @dateformat_str
 
+public format
+
 end # module