latin-ext: Subscript characters?

Our `latin` subset includes the first four superscript characters,

```
0x00B9 ¹ SUPERSCRIPT ONE
0x00B2 ² SUPERSCRIPT TWO
0x00B3 ³ SUPERSCRIPT THREE
0x2074 ⁴ SUPERSCRIPT FOUR
```

I don't think we should remove these from `latin` since that would break integrations.

However, if they exist, we should probably also enable access to the first 4 subscript characters,

```
0x2081 ₁ SUBSCRIPT ONE
0x2082 ₂ SUBSCRIPT TWO
0x2083 ₃ SUBSCRIPT THREE
0x2084 ₄ SUBSCRIPT FOUR
```

the other superscript numerals,

```
0x2070 ⁰ SUPERSCRIPT ZERO
0x2075 ⁵ SUPERSCRIPT FIVE
0x2076 ⁶ SUPERSCRIPT SIX
0x2077 ⁷ SUPERSCRIPT SEVEN
0x2078 ⁸ SUPERSCRIPT EIGHT
0x2079 ⁹ SUPERSCRIPT NINE
```

and the other subscript numerals,

```
0x2080 ₀ SUBSCRIPT ZERO
0x2085 ₅ SUBSCRIPT FIVE
0x2086 ₆ SUBSCRIPT SIX
0x2087 ₇ SUBSCRIPT SEVEN
0x2088 ₈ SUBSCRIPT EIGHT
0x2089 ₉ SUBSCRIPT NINE
```

but should be in our `latin-ext` rather than `latin`