chore: bump changelog to v0.9.3 (#63)

Tolerate leading '=' in SetFormula (#62)

* fix(formulas): strip leading equals from formula input

Allows users to input formulas with leading '=' as they would in Excel.
Prevents #NAME? errors by ensuring clean formula storage in OOXML.

The leading equals would nest inside the <f> element and break
evaluation if not stripped during SetFormula processing.

* chore: bump changelog to v0.9.2

Update CHANGELOG for v0.9.0 release

Add MIT license

docs: update CHANGELOG for v0.7.0

docs: add v0.6.1 changelog