FontSizePicker: Fix FontSizePicker Storybook control type and improve documentation

[himanshupathak95]

What?

Closes: #68553

This PR fixes an issue with the FontSizePicker component in Storybook where changing the units prop was causing an error. It also clarifies the documentation around the component's unitless mode.

Why?

When trying to change the units prop in Storybook, an error was occurring: allowedUnitValues.includes is not a function. This happened because clicking the Set object button in Storybook initialized the value as an empty object instead of an array. Additionally, the documentation didn't clearly explain when the units prop would actually have an effect.

How?

• Changes the Storybook control type for the units prop from the default to inline-check with predefined options
• Improves the README documentation to clearly explain the relationship between string/number font-size values and the unitless mode

Testing Instructions

• Run Storybook with npm run storybook:dev
• Navigate to the FontSizePicker component
• Verify you can select different unit options using the checkboxes without any errors
• Verify that the component's documentation explains that the units property only works when font sizes are specified as strings with units

Screencast

Screen.Recording.2025-02-26.at.12.43.14.mov

[himanshupathak95]

Hi, @t-hamano I have currently applied your fix and it seems to solve the issue but I am finding better possibilities.
This might not be the final solution but I think this is a discussion starter.
I will be updating the README and the storybook in the following commits.

Meanwhile, if we find any better solutions, we can explore them here.

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[t-hamano]

LGTM 👍