Prefixed

**Changes**
- Add [Significant digit support](https://prefixed.readthedocs.io/en/stable/#significant-digits)
- `'H'` (SI), `'K'` (IEC), and `'M'` (IEC short) presentation types treat precision as significant digits
- Trailing zeros are dropped by default, use `''` to preserve
- `'j'` (IEC) presentation type replaced with `'k'`
- `'j'` still works, but is deprecated
- `'J'` (IEC short) presentation type replaced with `'m'`
- `'J'` still works, but is deprecated
- Dropped support for Python 3.4

**Housekeeping**
- Moved to GitHub Actions
- Rename master to main

**Changes**
- No longer accept arbitrary characters after prefix in input strings
- Accept a single space between value and prefix in input strings

**Bug Fixes**
- Base for IEC values was incorrect in documentation

**Changes**
- Make import of Float more explicit in examples

**New Features**
- Added margin to format specification

Initial beta release