Add useTheme hook for runtime Polaris tokens access

[aaronccasanova]

Rework of #10229.

• To enhance clarity and avoid ambiguity, we have reintroduced the convention of prefixing all theme related objects and types with meta. While I was trying to steer us away from this convention, keeping it establishes consistency and makes it easier to differentiate between theme constructs with and without metadata.

• Introduced a new createThemeVariant utility, which accepts a theme partial and constructs a complete theme object. This utility simplifies the process of creating theme variants and ensures that all required properties are included.

• The AppProvider has been updated to incorporate a new ThemeContext.Provider and associated useTheme hook. This provider utilizes the createThemeVariant utility to expose the active theme to child components. This eliminates the need for several previously proposed exports and hooks, including themeVars, useThemeName, useThemeVarDecl, and useCSSDecl.

Before:

import {themeVars} from '@shopify/polaris';

themeVars.color['color-bg'] // 'var(--p-color-bg)'

import {useThemeVarDecl} from '@shopify/polaris';

function App() {
  const colorBgDecl = useThemeVarDecl('--p-color-bg');

  colorBgDecl // { status: 'success', value: '#123', ... }
}

After:

import {useTheme} from '@shopify/polaris';

function App() {
  const theme = useTheme()

  theme.color['color-bg'] // '#123'
}