feat: add defaultMode to specify initial keyboard layout on focus

[luizfern12]

Description

Added defaultMode to onscreen_keyboard_text_field and onscreen_keyboard_text_form_field.
Also added it to layout so the textFields can default to the correct user defined mode when defaultMode isn't specified on a textField.

Added tests for defaultMode.

This change aims to add a easier way of switching the keyboard mode without having to use setModeNamed manually for every textField.

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

• Checked for regressions using the provided sample.
• Tested locally with an modified version of the provided sample implementing the defaultMode field on the textFields.

Test Configuration:

• Package Version: 0.4.3+2
• Flutter SDK: 3.38.9
• Operating System: Linux 6.18.6-arch1-1 (Arch Linux)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[albinpk]

Thanks for the PR. This is a nice improvement.

I have a couple of suggestions:

• I think the defaultMode field on KeyboardLayout can be avoided. The default mode can simply be the first entry in the modes map, as it works today. Adding a separate defaultMode forces users to either always specify it or ensure the modes contain a specific key "default", which adds extra constraints.
• For the text field widgets, the property name defaultMode might be a bit generic. Using a more explicit name that clearly refers to the on-screen keyboard mode would improve clarity.

The approach makes sense; I just think these tweaks would simplify the API.

[luizfern12]

I'm leaving home right now, when i come back i'll look into implementing your suggestions:

About the naming, what do you think of 'OnscreenKeyboardMode'?