[EPIC] change SQL numeric literals from float to decimal

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Currently DataFusion interprets all non-integer numeric literals as floats, but they should be decimals instead.

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List of related issues. Probably all of these should be fixed before the actual switch to decimals is committed.
- [x] #3437
- [x] #3478
- [x] #3479
- [x] #3487
- [x] https://github.com/apache/arrow-datafusion/issues/4072
- [x] #3394
- [x] https://github.com/apache/arrow-datafusion/issues/4103
- [ ] https://github.com/apache/datafusion/issues/14612

This is an EPIC and names no files or tests. Start by reviewing the checked related issues and the remaining #14612, then locate DataFusion’s SQL numeric-literal parsing and typing entry points; done means the listed prerequisites are resolved and non-integer numeric literals consistently use decimals without regressions.

Written by the indexing model from the issue text.