fix(Dropdown): backspace with text selection will also remove selected element

[silviuaavram]

In the case of multiple search Dropdown, if you have selected items and an input value in the search, if you select first n characters and hit Backspace, you get the characters removed and also the last selected item.

It should remove the first selected item only when cursor is at the start position of the search and without any character selection.

[DustyTheBot]

Perf comparison

Status	Scenario	Fluent TPI	Fabric TPI	Ratio	Iterations	Ticks
🔧	Avatar.Fluent	0.65	0.5	1.3:1	2000	1299
🎯	Button.Fluent	0.14	0.19	0.74:1	1000	142
🔧	Checkbox.Fluent	0.84	0.31	2.71:1	1000	842
🔧	Dialog.Fluent	0.38	0.19	2:1	5000	1898
🔧	Dropdown.Fluent	3.54	0.38	9.32:1	1000	3537
🔧	Icon.Fluent	0.14	0.04	3.5:1	5000	699
🎯	Image.Fluent	0.06	0.08	0.75:1	5000	320
🔧	Slider.Fluent	1.61	0.38	4.24:1	1000	1609
🔧	Text.Fluent	0.06	0.02	3:1	5000	305
🦄	Tooltip.Fluent	0.09	17.04	0.01:1	5000	447

🔧 Needs work     🎯 On target     🦄 Amazing

Perf tests with no regressions

Scenario	Current PR Ticks	Baseline Ticks	Ratio
SliderMinimalPerf.default	2060	1594	1.29:1
Image.Fluent	320	279	1.15:1
Text.Fluent	305	266	1.15:1
AttachmentSlotsPerf.default	3866	3383	1.14:1
BoxMinimalPerf.default	242	217	1.12:1
ListNestedPerf.default	753	674	1.12:1
ProviderMinimalPerf.default	718	640	1.12:1
Button.Fluent	142	127	1.12:1
InputMinimalPerf.default	1024	927	1.1:1
RefMinimalPerf.default	195	178	1.1:1
ToolbarMinimalPerf.default	815	739	1.1:1
Dialog.Fluent	1898	1725	1.1:1
TableMinimalPerf.default	737	674	1.09:1
AvatarMinimalPerf.default	589	543	1.08:1
LayoutMinimalPerf.default	538	499	1.08:1
CustomToolbarPrototype.default	4086	3784	1.08:1
HeaderSlotsPerf.default	1356	1271	1.07:1
IconMinimalPerf.default	296	277	1.07:1
MenuButtonMinimalPerf.default	1525	1431	1.07:1
StatusMinimalPerf.default	316	295	1.07:1
Avatar.Fluent	1299	1217	1.07:1
TreeMinimalPerf.default	893	845	1.06:1
AlertMinimalPerf.default	593	567	1.05:1
AnimationMinimalPerf.default	513	490	1.05:1
ChatDuplicateMessagesPerf.default	581	553	1.05:1
LabelMinimalPerf.default	832	802	1.04:1
ListCommonPerf.default	827	794	1.04:1
TreeWith60ListItems.default	242	232	1.04:1
CarouselMinimalPerf.default	1966	1910	1.03:1
DropdownManyItemsPerf.default	423	411	1.03:1
HierarchicalTreeMinimalPerf.default	797	777	1.03:1
Tooltip.Fluent	447	433	1.03:1
AttachmentMinimalPerf.default	948	926	1.02:1
ChatMinimalPerf.default	1745	1710	1.02:1
FlexMinimalPerf.default	339	333	1.02:1
MenuMinimalPerf.default	2061	2014	1.02:1
ButtonSlotsPerf.default	617	608	1.01:1
DialogMinimalPerf.default	1727	1705	1.01:1
PortalMinimalPerf.default	213	211	1.01:1
TooltipMinimalPerf.default	629	622	1.01:1
CheckboxMinimalPerf.default	3833	3829	1:1
DropdownMinimalPerf.default	3443	3433	1:1
ItemLayoutMinimalPerf.default	1739	1740	1:1
ProviderMergeThemesPerf.default	1085	1083	1:1
AccordionMinimalPerf.default	202	204	0.99:1
ReactionMinimalPerf.default	3150	3179	0.99:1
ChatWithPopoverPerf.default	570	582	0.98:1
EmbedMinimalPerf.default	6079	6172	0.98:1
FormMinimalPerf.default	722	740	0.98:1
ListWith60ListItems.default	150	153	0.98:1
LoaderMinimalPerf.default	2688	2747	0.98:1
TextAreaMinimalPerf.default	3387	3446	0.98:1
ImageMinimalPerf.default	236	243	0.97:1
SplitButtonMinimalPerf.default	11637	11978	0.97:1
Dropdown.Fluent	3537	3630	0.97:1
HeaderMinimalPerf.default	401	419	0.96:1
RadioGroupMinimalPerf.default	426	448	0.95:1
ButtonMinimalPerf.default	111	120	0.93:1
SegmentMinimalPerf.default	1633	1783	0.92:1
Slider.Fluent	1609	1740	0.92:1
DividerMinimalPerf.default	853	936	0.91:1
Checkbox.Fluent	842	937	0.9:1
VideoMinimalPerf.default	715	805	0.89:1
TextMinimalPerf.default	287	327	0.88:1
Icon.Fluent	699	792	0.88:1
PopupMinimalPerf.default	312	357	0.87:1
ListMinimalPerf.default	324	379	0.85:1
GridMinimalPerf.default	808	967	0.84:1

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