BUG: Fix to_excel storing decimals as strings instead of numbers (issue #49598)

[michiel-de-muynck]

• closes BUG: Dataframe.to_excel treats decimal.Decimal as string instead of numeric type, the data in the Excel cell is formatted as a string, not a number #49598
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.

This is the same bug fix code as PR #50174, which went stale and had no tests. The author of that PR is no longer responding. This PR adds tests and release notes.

[mroeschke]

Thanks @michiel-de-muynck

[LorenzoMorelli]

Hi, thanks for your work!
When I export a dataframe to my PostgreSQL database with to_sql(...) using if_exists='replace', decimal.Decimal values are stored as TEXT instead of DECIMAL. Does this patch also fix this issue?

[michiel-de-muynck]

Hi Lorenzo, this only affects Excel, not PostgreSQL.

[asishm]

@mroeschke this is milestoned 3.0 but the whatsnew entry is in 2.3. does that need to be addressed (or backported)?

[mroeschke]

Oops the whatsnew entry should be moved to v3.0.0.rst. Could you open another PR to do that @michiel-de-muynck?