Feat: MaxLength Validation

[Vaibhav2002]

Fixes #143

[Vaibhav2002]

Hey @MohamedRejeb , there is this one issue which I couldn't understand how to fix
For example, I've set maxLength to 15, now after I have typed 15 or 14 characters, if I toggle Unordered/Ordered List, the app crashes with this kind of exception

java.lang.StringIndexOutOfBoundsException: begin 2, end 16, length 13
    at java.lang.String.checkBoundsBeginEnd(String.java:3942)
    at java.lang.String.substring(String.java:2467)
    at com.mohamedrejeb.richeditor.utils.AnnotatedStringExtKt.append-RcvT-LA(AnnotatedStringExt.kt:83)
    at com.mohamedrejeb.richeditor.utils.AnnotatedStringExtKt.append-RcvT-LA(AnnotatedStringExt.kt:24)
    at com.mohamedrejeb.richeditor.model.RichTextState.updateAnnotatedString(RichTextState.kt:622)
    at com.mohamedrejeb.richeditor.model.RichTextState.updateTextFieldValue(RichTextState.kt:574)
    at com.mohamedrejeb.richeditor.model.RichTextState.updateParagraphType(RichTextState.kt:458)
    at com.mohamedrejeb.richeditor.model.RichTextState.addUnorderedList(RichTextState.kt:393)
    at com.mohamedrejeb.richeditor.model.RichTextState.toggleUnorderedList(RichTextState.kt:383)

Any idea, how to fix this?

[MohamedRejeb]

I'll merge it and fix the error.