Rename prettyPrintedTwoSpaces to indentationTwoSpaces

[mattt]

Follow-up to #13

yyjson has an unfortunate API design where it has YYJSON_WRITE_PRETTY and YYJSON_WRITE_PRETTY_TWO_SPACES. As peer C flags, they compose awkwardly — one implies the other, but there's an implicit hierarchy of refinement (petty > pretty w/2 spaces).

This PR attempts to take the edge off of that tension with a heavy-handed application of Swift API naming guidelines. I'm not sure how successful we are in this regard (maybe we're worse off for creating unnecessary drift from underlying C API?) But some part of me just couldn't abide having both .prettyPrinted and .prettyPrintedTwoSpaces in the public API.

[Copilot]

Pull request overview

Renames the public 2-space pretty-printing option from prettyPrintedTwoSpaces to indentationTwoSpaces across the Swift API surface, aiming to better reflect the underlying yyjson flag behavior while aligning with Swift naming guidelines.

Changes:

• Renamed prettyPrintedTwoSpaces to indentationTwoSpaces in YYJSONSerialization.WritingOptions and YYJSONWriteOptions.
• Updated write-flag mapping logic and all test references to use the new option name.
• Updated README examples and option lists to reflect the new name.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
Tests/YYJSONTests/SerializationTests.swift	Renames tests and option usage to .indentationTwoSpaces.
Tests/YYJSONTests/EncoderTests.swift	Updates encoder tests to use .indentationTwoSpaces.
Tests/YYJSONTests/ConfigurationTests.swift	Updates configuration tests to use .indentationTwoSpaces.
Sources/YYJSON/Serialization.swift	Renames WritingOptions case and updates flag mapping checks.
Sources/YYJSON/Configuration.swift	Renames YYJSONWriteOptions case and updates documentation string.
README.md	Updates docs/examples to the new option name.

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