fix: input maxlength constraint should respect selection

[marshcat0]

What:

For a text input / textarea that has maxlength, when the value reachs the maxlength limit, and the value is selected, new text should still be inserted. Becase the new text will replace the selected text.

The old code did not take the selection into consideration, causing new text not inserted.

Why:

To match how browsers behave

How:

When calulate the space remaining for maxlength, add the selected range length to the space.

Checklist:

• Documentation
• Tests
• Ready to be merged
  

• According to the html spec, the selectionStart is less than or equal to selectionEnd, so inputRange.endOffset - inputRange.startOffset must greater than or equal 0, which meets our expectation.
• I want to reuse the maxlength test case, but the ternary expression added looks terrible. So feel free if you wannt to edit the test case :)

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