Add support for explicitly typed constants

[attackgoat]

I found this testing attackgoat/screen-13#68

Example shader

#version 460 core
#extension GL_EXT_shader_explicit_arithmetic_types_int8 : require
#extension GL_EXT_shader_explicit_arithmetic_types_int16 : require
#extension GL_EXT_shader_explicit_arithmetic_types_int32 : require
#extension GL_EXT_shader_explicit_arithmetic_types_int64 : require
#extension GL_EXT_shader_explicit_arithmetic_types_float16 : require
#extension GL_EXT_shader_explicit_arithmetic_types_float32 : require
#extension GL_EXT_shader_explicit_arithmetic_types_float64 : require

const int8_t INT8 = int8_t(1);
const int16_t INT16 = int16_t(1);
const int32_t INT32 = int32_t(1);
const int64_t INT64 = int64_t(1);

const uint8_t UINT8 = uint8_t(1);
const uint16_t UINT16 = uint16_t(1);
const uint32_t UINT32 = uint32_t(1);
const uint64_t UINT64 = uint64_t(1);

const float16_t FLOAT16 = float16_t(1.0);
const float32_t FLOAT32 = float32_t(1.0);
const float64_t FLOAT64 = float64_t(1.0);

void main() {
}

These constants probably don't need to be parsed, invalidating this change. If they should be parsed then maybe half could help with the f16 representation.

[PENGUINLIONG]

Thank you!