Rename ValueSupport subtypes to more specific names

[juliohm]

Currently the subtypes of ValueSupport are Discrete and Continuous as described here:

Distributions.jl/docs/src/types.md

Lines 34 to 50 in b356da0

	### ValueSupport
	```@docs
	Distributions.ValueSupport
	```
	The `ValueSupport` sub-types defined in `Distributions.jl` are:
	```@docs
	Distributions.Discrete
	Distributions.Continuous
	```
	**Type** | **Default element type** | **Description** | **Examples**
	--- | --- | --- | ---
	`Discrete` | `Int` | Samples take countably many values | $\{0,1,2,3\}$, $\mathbb{N}$
	`Continuous` | `Float64` | Samples take uncountably many values | $[0, 1]$, $\mathbb{R}$

These names are too generic to export, and conflict with other mathematical packages.

Would you be ok with renaming these two to DiscreteSupport and ContinuousSupport?

I can work on a PR that includes a deprecation warning.

[devmotion]

Is there an actual problem/conflict with some other package? My initial feeling is that we should only make such a change if it's worth it.