[Bug]: immutable-state detector incorrectly claims strings can be made immutable

[area]

Describe the issue:

According to the solidity docs while strings can be declared constant, they can not be declared immutable at this time. This detector, however, asks for strings to be declared immutable as if they were supported. Doing so causes the solidity compilation to fail.

Code example to reproduce the issue:

pragma solidity 0.8.17;

contract X {
  bool public immutable boolVar;
  string public stringVar;


  constructor(bool _boolVar, string memory _stringVar) {
    boolVar = _boolVar;
    stringVar = _stringVar;
  }
}

Version:

0.9.2

Relevant log output:

No response

[0xalpharush]

This wasn't accounted for when reusing the _valid_type helper function from the constant detector. Just need to filter out string types here

slither/slither/detectors/variables/unchanged_state_variables.py

Lines 120 to 123 in 4c976d5

	elif (
	v in constructor_variables_written or v in variables_initialized
	) and version.parse(self.compilation_unit.solc_version) >= version.parse(
	"0.6.5"