[@mantine/vanilla-extract] theme-to-vars: Fix headings' vars

[soudai-s]

Generated CSS Variables about headings by theme-to-vars were unintended values are follows
Because the keys of mergedTheme.headings are not h1 ~ h6, but fontFamily, fontWeight, sizes.

{
  fontFamily: {
    fontSize: 'var(--mantine-fontFamily-font-size)',
    lineHeight: 'var(--mantine-fontFamily-line-height)',
    fontWeight: 'var(--mantine-fontFamily-font-weight)'
  },
  fontWeight: {
    fontSize: 'var(--mantine-fontWeight-font-size)',
    lineHeight: 'var(--mantine-fontWeight-line-height)',
    fontWeight: 'var(--mantine-fontWeight-font-weight)'
  },
  sizes: {
    fontSize: 'var(--mantine-sizes-font-size)',
    lineHeight: 'var(--mantine-sizes-line-height)',
    fontWeight: 'var(--mantine-sizes-font-weight)'
  }
},
{
  fontFamily: {
    fontSize: 'var(--mantine-fontFamily-font-size)',
    lineHeight: 'var(--mantine-fontFamily-line-height)',
    fontWeight: 'var(--mantine-fontFamily-font-weight)'
  },
  fontWeight: {
    fontSize: 'var(--mantine-fontWeight-font-size)',
    lineHeight: 'var(--mantine-fontWeight-line-height)',
    fontWeight: 'var(--mantine-fontWeight-font-weight)'
  },
  sizes: {
    fontSize: 'var(--mantine-sizes-font-size)',
    lineHeight: 'var(--mantine-sizes-line-height)',
    fontWeight: 'var(--mantine-sizes-font-weight)'
  }
},
{
  fontFamily: {
    fontSize: 'var(--mantine-fontFamily-font-size)',
    lineHeight: 'var(--mantine-fontFamily-line-height)',
    fontWeight: 'var(--mantine-fontFamily-font-weight)'
  },
  fontWeight: {
    fontSize: 'var(--mantine-fontWeight-font-size)',
    lineHeight: 'var(--mantine-fontWeight-line-height)',
    fontWeight: 'var(--mantine-fontWeight-font-weight)'
  },
  sizes: {
    fontSize: 'var(--mantine-sizes-font-size)',
    lineHeight: 'var(--mantine-sizes-line-height)',
    fontWeight: 'var(--mantine-sizes-font-weight)'
  }
}

So I have fixed this problem.

[rtivital]

Thanks!