Add warning to not implement sprout-generated visitor interfaces

[alancai98]

Relevant Issues

• Closes [sprout] Add warning/@OptIn option for sprout-generated interfaces #1405.
• Copies over the kdoc warning from Add opt-in for sprout-generated interfaces #1406.

Description

Similar to #1406 but just adds the warning to not implement the sprout-generated visitor interfaces. This will currently be applied for the AstVisitor and PlanVisitor.

// AstVisitor.kt
/**
 * WARNING: This interface should not be implemented or extended by code outside of this library.
 * Please extend [AstBaseVisitor].
 */
public interface AstVisitor<R, C> {
	// rest of code
}

// PlanVisitor.kt
/**
 * WARNING: This interface should not be implemented or extended by code outside of this library.
 * Please extend [PlanBaseVisitor].
 */
public interface PlanVisitor<R, C> {
	// rest of code
}

Other Information

• Updated Unreleased Section in CHANGELOG: [YES]

• Any backward-incompatible changes? [NO]

• Any new external dependencies? [NO]

• Do your changes comply with the Contributing Guidelines
  and Code Style Guidelines? [YES]

License Information

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[github-actions]

Conformance comparison report

	Base (d059c81)	54bba32	+/-
% Passing	92.54%	92.54%	0.00%
✅ Passing	5384	5384	0
❌ Failing	434	434	0
🔶 Ignored	0	0	0
Total Tests	5818	5818	0

Number passing in both: 5384

Number failing in both: 434

Number passing in Base (d059c81) but now fail: 0

Number failing in Base (d059c81) but now pass: 0

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.16%. Comparing base (d059c81) to head (6f667e9).

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #1413   +/-   ##
=========================================
  Coverage     73.16%   73.16%           
  Complexity     2393     2393           
=========================================
  Files           247      247           
  Lines         17627    17627           
  Branches       3178     3178           
=========================================
  Hits          12896    12896           
  Misses         3854     3854           
  Partials        877      877           

Flag	Coverage Δ
CLI	11.82% <ø> (ø)
EXAMPLES	80.07% <ø> (ø)
LANG	81.06% <ø> (ø)

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