feat: Implement overriding API methods and correct inconsistent naming

[LeoniePhiline]

This is a successor to https://github.com/allan2/dotenvy/pull/17 and the preferred alternative to https://github.com/allan2/dotenvy/pull/46.

This PR

• Adds overriding variants to all relevant public API methods.
• Fixes documentation.
• Adds missing test for from_read().
• Tests non-overriding behavior with greater precision.
• Tests overriding behavior correctly.
• Fixes some inconsistent test formatting.
• Difference from #46: Fixes inconsistent naming. See discussion in #46 for details.

[LeoniePhiline]

@sonro PTAL

Changes: https://github.com/allan2/dotenvy/compare/392628a9610705d0948c0191ce520f2dc5189935..138476842bb73e423358e10943040461de94a1dd

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