Fix exponentiation precedence for compound exponents

[cknitt]

Fixes #8549.

Summary

• Preserve parentheses around multiplication, division, and modulo expressions used as the right operand of exponentiation.
• Add regression coverage for float multiplication, division, and modulo, plus bigint multiplication.

Root cause

The JavaScript printer assigned exponentiation a right-operand precedence of 12, equal to multiplication, division, and modulo. Because parentheses are only added when the required precedence is greater than the child expression's precedence, expressions such as a ** (b / c) were emitted as a ** b / c, changing their meaning.

Requiring precedence 13 on the right preserves compound exponents while retaining JavaScript's right-associative a ** b ** c output.

Regression history and backport

This regression was introduced by #7153 when float and integer exponentiation switched from Math.pow calls to native JavaScript **.

The change was included in ReScript 12.0.0, so all 12.x releases are affected. This fix should therefore be backported to the maintained 12.x release branch.

Validation

• make test
• make test-syntax
• make checkformat
• git diff --check

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.84%. Comparing base (65cf698) to head (254cb45).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #8550   +/-   ##
=======================================
  Coverage   74.84%   74.84%           
=======================================
  Files         477      477           
  Lines       63331    63331           
=======================================
  Hits        47398    47398           
  Misses      15933    15933           

Files with missing lines	Coverage Δ
compiler/core/js_op_util.ml	56.32% <100.00%> (ø)

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commit: 254cb45