Macos deltas composing fix

[Renzo-Olivares]

This fixes an issue where the incorrect composing region was sent as a delta to the framework, causing no delta to be created on the framework side because the delta range did not exist. Sending the correct composing region also fixes inserting of special characters with opt+n / opt+u

Also updated testOperationsWithDelta test
Old:

  [plugin setMarkedText:@"marked text" selectedRange:NSMakeRange(0, 1)];

  deltaToFramework = @{
    @"oldText" : @"text to insert",
    @"deltaText" : @"marked text",
    @"deltaStart" : @(14),
    @"deltaEnd" : @(25),
    @"selectionBase" : @(25),
    @"selectionExtent" : @(25),
    @"selectionAffinity" : @"TextAffinity.upstream",
    @"selectionIsDirectional" : @(false),
    @"composingBase" : @(14),
    @"composingExtent" : @(25),
  };

New:

  [plugin setMarkedText:@"marked text" selectedRange:NSMakeRange(0, 1)];

  deltaToFramework = @{
    @"oldText" : @"text to insert",
    @"deltaText" : @"marked text",
    @"deltaStart" : @(14),
    @"deltaEnd" : @(14),
    @"selectionBase" : @(25),
    @"selectionExtent" : @(25),
    @"selectionAffinity" : @"TextAffinity.upstream",
    @"selectionIsDirectional" : @(false),
    @"composingBase" : @(14),
    @"composingExtent" : @(25),
  };

When setMarkedText is called at this point in the test, it is the beginning of composition so the text will be inserted at the collapsed selection. deltaStart and deltaEnd was previously set to the range of the new composing region, but this is incorrect because the range of (14,25) does not exist in the text before this delta is applied.

Fixes flutter/flutter#101013

Added testComposingWithDelta

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[justinmc]

LGTM 👍

Thanks for the really thorough test, it's clearly needed.