Refactor Pattern value/format accessors into base class

[ddmitv]

Refactors pattern value access and formatting so user [[format]] attribute handling is centralized in Pattern::getFormattedValue() and Pattern::toString(), while per-pattern formatting moves into defaultToDisplayValue() and defaultToString() fallbacks. For accessing pattern's value added getRawValue for pre-[[transform]] access.

Make [[format]] functions to always receive the untransformed value (via getRawValue()) instead of the transformed one for scalar types.
Previously, fn format_field(u32 x) { ... } and fn transform_field(u32 x) { ... } applied on field u32 field [[format("format_field"), transform("transform_field")]] will apply transform_field() before passing the value to format_field(). But this is not the same for pattern types like struct, meaning for pattern values it will not apply transform_field(), and direclty pass pattern's value to format_field function. This change fixes the inconsistent behavior by never applying [[transform]] before [[format]].
I ran tests on the ImHex-Pattern repo with code from this PR and confirmed that existing hexpats do not rely on this behavior.

API changes

• getValue() const override -> getRawValue() const override + drop transformValue(...).
• toString() override -> defaultToString() const override.
• formatDisplayValue() override -> defaultToDisplayValue() const override.
• callUserFormatFunc(literal, fromCast) -> callUserFormatFunc(fromCast).
• getValue() and toString() are no longer virtual.

Due to these breaking API changes, the main ImHex repo will not compile because it relied on them. I opened a draft PR for ImHex repo to adapt to these changes when this PR is merged - WerWolv/ImHex#2910