Editorial: Refactor PartitionPattern for simplicity and comprehensibility

[gibson042]

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• Adds a prose description explaining treatment of brace-wrapped placeholders and adjacent literal text in converting a String pattern into an equivalent List of Records.
• Renames spec aliases (beginIndex → placeholderStart, endIndex → placeholderEnd, p → placeholderName).
• Adopts ECMA-262 phrasing conventions (e.g., "the substring of S from inclusiveStart [to exclusiveEnd]" and </≤/≥/> operators).
• Reduces unnecessary repetition (e.g., "If beginIndex > nextIndex, then Let literal be the substring of pattern from nextIndex to beginIndex and Append the Record { [[Type]]: "literal", [[Value]]: literal } to result." → "Let literal be the substring of pattern from placeholderEnd + 1 to placeholderStart and If literal is not the empty String, then Append the Record { [[Type]]: "literal", [[Value]]: literal } to result.").
• Makes more obvious the omission of zero-length literal Records.
• Increases alias reference locality (e.g., checking for a literal part between a previous placeholderEnd } and current placeholderStart { before advancing placeholderEnd for the current placeholder part).

[gibson042]

@ryzokuken ping