feat(text_input): add select_range method and Task function

[Dellareti]

Adds a public select_range(start, end) method to text_input::State and a corresponding Task function, following the same pattern as select_all.

Depends on: Dellareti/iced#1 (adds select_range to the iced TextInput operation trait)

Note: The iced submodule reference will need to be updated once the iced PR is merged.

Related: pop-os/cosmic-files#181

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[Dellareti]

Adds a public select_range(start, end) method to text_input::State and a corresponding Task function, following the same pattern as select_all

Depends on: pop-os/iced#260 (adds select_range to the iced TextInput operation trait)

Note: The iced submodule reference will need to be updated once the iced PR is merged.