[SwiftOperators] Expose precedence-group comparison on OperatorTable

[devdattatalele]

Summary

Adds a public API to OperatorTable that reports the relationship between two precedence groups, so callers can determine whether one group has higher, lower, or unrelated precedence to another without re-implementing the traversal:

public func precedence(
  of firstGroupName: PrecedenceGroupName,
  relativeTo secondGroupName: PrecedenceGroupName,
  referencedFrom syntax: Syntax,
  errorHandler: OperatorErrorHandler = { throw \$0 }
) rethrows -> Precedence

The Precedence enum (and its flipped accessor) are also made public so callers can consume the result.

Motivation

This resolves #2258. The existing use case is swift-format PR apple/swift-format#647, where a caller wanted to compare the precedence of two operators. That comparison lived on the internal PrecedenceGraph.precedence(relating:to:startSyntax:endSyntax:) helper, so callers had no supported way to obtain the information short of re-implementing the traversal.

This PR does not change the algorithm — it lifts the existing internal capability to the public surface.

Design notes

• The public method is added to OperatorTable+Semantics.swift because that file already declares public import SwiftSyntax, so its public API can accept Syntax in the signature (needed to preserve the same error-reporting semantics as the internal helper).
• The referencedFrom: parameter mirrors the pattern used by the existing internal lookupOperatorPrecedenceGroupName(_:referencedFrom:errorHandler:) helper on OperatorTable. Callers that already have a syntax node (e.g. the operator token being compared) simply pass it through.
• The errorHandler default matches the rest of the module: { throw \$0 } (re-throw the underlying OperatorError).

Test plan

Three unit tests in OperatorTableTests:

• testPrecedenceGroupComparison exercises the new API on the standard operator table across four scenarios:
  • Direct higherThan relationship (MultiplicationPrecedence vs AdditionPrecedence).
  • The flipped direction returning .lowerThan.
  • The same-group case returning .unrelated.
  • Two groups that both sit above TernaryPrecedence with no other links between them (DefaultPrecedence vs LogicalDisjunctionPrecedence) returning .unrelated.
• testPrecedenceGroupFlipped covers Precedence.flipped for all three cases.

Files changed

• Sources/SwiftOperators/PrecedenceGraph.swift: mark Precedence and Precedence.flipped public — +2 / -2.
• Sources/SwiftOperators/OperatorTable+Semantics.swift: add precedence(of:relativeTo:referencedFrom:errorHandler:) — +29 / -0.
• Tests/SwiftOperatorsTest/OperatorTableTests.swift: add two new tests — +55 / -0.

Total: +86 / -2.

Resolves #2258.