Fix/accessibility labels

[gabrielezereik]

Add semantics support for header navigation arrows

This PR adds accessibility support to the header navigation arrows by introducing two new optional parameters in PickerHeaderSettings:
previousButtonSemanticsLabel — semantic label for the "previous" arrow button
nextButtonSemanticsLabel — semantic label for the "next" arrow button
These labels are passed down to a Semantics widget wrapping each IconButton in HeaderArrows, with button: true and excludeSemantics: true to ensure screen readers announce the custom label instead of the default "icon" description.

Motivation

Without explicit semantics, screen readers (e.g. TalkBack, VoiceOver) would announce the arrows as "icon", providing no meaningful context to visually impaired users. With this change, developers can provide localized, descriptive labels for these controls.
Usage

MonthPickerDialogSettings(
  headerSettings: PickerHeaderSettings(
    previousButtonSemanticsLabel: 'Previous month',
    nextButtonSemanticsLabel: 'Next month',
  ),
)

[Macacoazul01]

Tks for the PR. Just gonna check something about the semantics widget and then merge

[Macacoazul01]

@gabrielezereik im gonna add it to the library without changing the default fallback for now (MaterialLocalizations.of(context).nextMonthTooltip and previousMonthTooltip) and lets see if this causes a problem to someone's project.