fix(react-motions-preview): fix easing token values; match original names & structure

[robertpenner]

Context

• Motion tokens (duration and easing) are defined in @fluentui/tokens, but are not exported there for direct consumption in JavaScript. Only the CSS variable names are exported.
• The motion tokens were copied into @fluentui/react-motions-preview to allow JavaScript consumption. This was deemed the least intrusive solution after a discussion of other possible approaches.
• The copied motion tokens deviated from the original in terms of some numerical values, names and structure.

Previous Behavior

• Easing curve token values in @fluentui/react-motions-preview are incorrect (same errors as in #29546)
• Easing curve token names do not match those in @fluentui/tokens (easing___ vs. curve___)
• Duration and easing tokens are combined in one flat set, whereas the original has them grouped in durations and curves objects

New Behavior

• Easing curve token values are in sync with packages/tokens/src/global/curves.ts
• Easing curve tokens are named curve___ as in the original
• The original's durations and curves objects are restored. This supports use cases like motion configurators where the durations and curves options are listed in dropdowns.
• The combined motionTokens object is kept as an combined, flat lookup. This can be more convenient for some use cases.

Related Issue(s)

• Fixes #31191

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