[Typography] Small improvements

[oliviertassinari]

• Switch the default variant between typography v1 and v2.
• Don't apply the scale twice
• Round the letter spacing value
• Makes Material-UI less opinionated regarding the letter-spacing value. I think that the specification is promoting a too strong letter-spacing value, at least for a minor release. I'm softening the change.

[eps1lon]

I'm really lost how it is less opinionated to apply letter-spacing you seem appropriate instead of the letter spacing that is defined in the spec. I also fail to see why this is a concern for versioning. We're adding a new feature that is completely backwards compatible. What do you mean by "softening"?

It's the second time you want to change this. Do we want to implement the spec or do we interpret the spec?

[eps1lon]

That's a breaking change. What's the purpose of this?

[oliviertassinari]

How is this a breaking change?

[oliviertassinari]

do we interpret the spec?

We do interpret the specification, as Google is doing with all its products. Even the Material Design specification website and the MDC Web implementations are interpreting the typography spec. There are two topics here.
The first one is around keeping 14px as the default font size when using the Typography component. To achieve that, we need to switch body 1 with body 2. Also, I think that we should use the next typography for the regression tests.
The second topic is around not changing the letter spacing too quickly. We can always change the letter spacing in v4.0.0 to a final value. I don't think that we should be updating our implementation faster than Google is updating its products. I haven't seen them executing on the new letter spacing yet. One step at the time.