Add missing operator and assignment expressions

[techouse]

I have recently struggled to clean up a package that I maintain using Expression operators instead of using Code blocks full of strings, i.e. Code('$a ^ $b') and have discovered that quite a few operators are missing from the Expression class. So I've added all the missing operators and assignments found on https://dart.dev/language/operators.

• added operators

  method	generates
  Expression.operatorIntDivide	this ~/ other
  Expression.operatorUnaryPrefixIncrement	++this
  Expression.operatorUnaryPostfixIncrement	this++
  Expression.operatorUnaryMinus	-this
  Expression.operatorUnaryPrefixDecrement	--this
  Expression.operatorUnaryPostfixDecrement	this--
  Expression.operatorBitwiseAnd	this & other
  Expression.operatorBitwiseOr	this \| other
  Expression.operatorBitwiseXor	this ^ other
  Expression.operatorUnaryBitwiseComplement	~this
  Expression.operatorShiftLeft	this << other
  Expression.operatorShiftRight	this >> other
  Expression.operatorShiftRightUnsigned	this >>> other

• added assignments

  method	generates
  Expression.addAssign	this += other
  Expression.subtractAssign	this -= other
  Expression.multiplyAssign	this *= other
  Expression.divideAssign	this /= other
  Expression.intDivideAssign	this ~/= other
  Expression.euclideanModuloAssign	this %= other
  Expression.shiftLeftAssign	this <<= other
  Expression.shiftRightAssign	this >>= other
  Expression.shiftRightUnsignedAssign	this >>>= other
  Expression.bitwiseAndAssign	this &= other
  Expression.bitwiseXorAssign	this ^= other
  Expression.bitwiseOrAssign	this \|= other

• deprecated the misnamed Expression.operatorSubstract method and created a correctly named Expression.operatorSubtract method.

• [x] I’ve reviewed the contributor guide and applied the relevant portions to this PR.