feat(FlexItem): allow custom values as size

[silviuaavram]

Allows any string passed as size prop to FlexItem and uses that value as fallback if none of the existing size values are passed.

  /** Defines size of the item. */
  size?: 'size.half' | 'size.quarter' | 'size.small' | 'size.medium' | 'size.large' | string

Values such as size={'30rem'} or size={'400px'} should work now and would end up as flex-basis on the FlexItem.

Fixes #2110

[DustyTheBot]

Perf comparison

Status	Scenario	Fluent TPI	Fabric TPI	Ratio	Iterations	Ticks
🔧	Avatar.Fluent	0.55	0.41	1.34:1	2000	1094
🔧	Button.Fluent	1.21	0.16	7.56:1	1000	1207
🔧	Checkbox.Fluent	1.4	0.29	4.83:1	1000	1400
🔧	Dialog.Fluent	0.32	0.17	1.88:1	5000	1585
🔧	Dropdown.Fluent	4.6	0.52	8.85:1	1000	4597
🔧	Icon.Fluent	0.25	0.03	8.33:1	5000	1271
🔧	Image.Fluent	0.12	0.08	1.5:1	5000	582
🔧	Slider.Fluent	1.94	0.4	4.85:1	1000	1935
🦄	Text.Fluent	0.05	0.16	0.31:1	5000	271
🦄	Tooltip.Fluent	0.42	18.57	0.02:1	5000	2093

🔧 Needs work     🎯 On target     🦄 Amazing

Perf tests with no regressions

Scenario	Current PR Ticks	Baseline Ticks	Ratio
BoxMinimalPerf.default	289	231	1.25:1
HeaderSlotsPerf.default	1778	1525	1.17:1
Dropdown.Fluent	4597	3950	1.16:1
ButtonMinimalPerf.default	1452	1265	1.15:1
Image.Fluent	582	509	1.14:1
VideoMinimalPerf.default	750	662	1.13:1
AttachmentMinimalPerf.default	1112	997	1.12:1
Text.Fluent	271	242	1.12:1
DropdownManyItemsPerf.default	692	627	1.1:1
FormMinimalPerf.default	996	904	1.1:1
ChatWithPopoverPerf.default	678	620	1.09:1
CustomToolbarPrototype.default	5041	4620	1.09:1
DialogMinimalPerf.default	1891	1763	1.07:1
DropdownMinimalPerf.default	4660	4358	1.07:1
PortalMinimalPerf.default	304	285	1.07:1
ChatMinimalPerf.default	1749	1652	1.06:1
GridMinimalPerf.default	905	854	1.06:1
LabelMinimalPerf.default	2193	2093	1.05:1
MenuButtonMinimalPerf.default	1831	1745	1.05:1
Checkbox.Fluent	1400	1338	1.05:1
AccordionMinimalPerf.default	218	209	1.04:1
AlertMinimalPerf.default	605	584	1.04:1
AttachmentSlotsPerf.default	3567	3421	1.04:1
InputMinimalPerf.default	1285	1238	1.04:1
ListCommonPerf.default	1323	1267	1.04:1
ProviderMergeThemesPerf.default	1460	1414	1.03:1
StatusMinimalPerf.default	988	961	1.03:1
EmbedMinimalPerf.default	6862	6758	1.02:1
MenuMinimalPerf.default	2344	2302	1.02:1
TextMinimalPerf.default	299	294	1.02:1
TreeMinimalPerf.default	1002	980	1.02:1
Button.Fluent	1207	1188	1.02:1
ListMinimalPerf.default	790	784	1.01:1
SplitButtonMinimalPerf.default	14571	14494	1.01:1
Slider.Fluent	1935	1913	1.01:1
RadioGroupMinimalPerf.default	480	478	1:1
ChatDuplicateMessagesPerf.default	559	567	0.99:1
FlexMinimalPerf.default	382	387	0.99:1
SegmentMinimalPerf.default	1512	1532	0.99:1
TextAreaMinimalPerf.default	3724	3745	0.99:1
ItemLayoutMinimalPerf.default	2112	2160	0.98:1
ReactionMinimalPerf.default	3068	3131	0.98:1
CheckboxMinimalPerf.default	7287	7538	0.97:1
ImageMinimalPerf.default	658	676	0.97:1
LayoutMinimalPerf.default	600	617	0.97:1
ToolbarMinimalPerf.default	845	870	0.97:1
Avatar.Fluent	1094	1124	0.97:1
CarouselMinimalPerf.default	1933	2021	0.96:1
Icon.Fluent	1271	1318	0.96:1
ButtonSlotsPerf.default	1844	1944	0.95:1
DividerMinimalPerf.default	1012	1070	0.95:1
RefMinimalPerf.default	188	198	0.95:1
AvatarMinimalPerf.default	593	633	0.94:1
LoaderMinimalPerf.default	2925	3112	0.94:1
TooltipMinimalPerf.default	2630	2804	0.94:1
Dialog.Fluent	1585	1732	0.92:1
PopupMinimalPerf.default	433	474	0.91:1
ProviderMinimalPerf.default	683	757	0.9:1
SliderMinimalPerf.default	2197	2505	0.88:1
AnimationMinimalPerf.default	457	525	0.87:1
IconMinimalPerf.default	1197	1372	0.87:1
Tooltip.Fluent	2093	2441	0.86:1
TableMinimalPerf.default	653	786	0.83:1
HierarchicalTreeMinimalPerf.default	797	1020	0.78:1
HeaderMinimalPerf.default	419	544	0.77:1

Generated by 🚫 dangerJS

[mnajdova]

Please add changelog entry before merging