Capitalization in EnzymeCore

> [!WARNING]
> The following propositions are very breaking and not essential, so weigh them carefully.

Since a breaking release may be on the horizon, here are some of my gripes about capitalization and style. My reference is the official (albeit concise) [Julia style guide](https://docs.julialang.org/en/v1/manual/style-guide).

## Mode setters

> [Use naming conventions consistent with Julia `base/`](https://docs.julialang.org/en/v1/manual/style-guide/#Use-naming-conventions-consistent-with-Julia-base/):
> - modules and type names use capitalization and camel case: `module SparseArrays`, `struct UnitRange`.
> - functions are lowercase (`maximum`, `convert`) and, when readable, with multiple words squashed together (`isequal`, `haskey`). When necessary, use underscores as word separators.

Mode setters are functions. At the moment, some are already lowercase

```julia
clear_err_if_func_written
clear_runtime_activity
set_abi
set_err_if_func_written
set_runtime_activity
```

but some are uppercase and could be renamed (possibly while keeping the deprecated version around with a warning):

```julia
Combined => combined_mode
NoPrimal => noprimal_mode
Split => split_mode # split already exists
WithPrimal => with_primal
```

## Mode objects

> [Avoid confusion about whether something is an instance or a type](https://docs.julialang.org/en/v1/manual/style-guide/#Avoid-confusion-about-whether-something-is-an-instance-or-a-type)

To me, the following choices in Enzyme seemed rather confusing at first:

```julia
julia> ForwardMode isa Type
true

julia> Forward isa Type
false

julia> Forward isa ForwardMode
true
```

If renaming were easy, I would suggest either of the following conventions instead:

1. make mode instances all-uppercase like other module-level constants (recommendation found in [BlueStyle](https://github.com/JuliaDiff/BlueStyle?tab=readme-ov-file#global-variables) and [SciMLStyle](https://github.com/SciML/SciMLStyle?tab=readme-ov-file#globals-should-be-avoided-whenever-possible)

```julia
Forward => FORWARD
ForwardWithPrimal => FORWARD_WITH_PRIMAL
Reverse => REVERSE
ReverseWithPrimal => REVERSE_WITH_PRIMAL
ReverseHolomorphic => REVERSE_HOLOMORPHIC
ReverseHolomorphicWithPrimal => REVERSE_HOLOMORPHIC_WITH_PRIMAL
ReverseSplitNoPrimal => REVERSE_SPLIT_NO_PRIMAL
ReverseSplitWithPrimal => REVERSE_SPLIT_WITH_PRIMAL
```

2. obtain mode instances as the output of constructors, i.e. add parentheses

```julia
Forward => Forward()
ForwardWithPrimal => ForwardWithPrimal()
Reverse => Reverse()
ReverseWithPrimal => ReverseWithPrimal()
ReverseHolomorphic => ReverseHolomorphic()
ReverseHolomorphicWithPrimal => ReverseHolomorphicWithPrimal()
ReverseSplitNoPrimal => ReverseSplitNoPrimal()
ReverseSplitWithPrimal => ReverseSplitWithPrimal()
```

However I realize that this would break everyone's code everywhere, so this is obviously low priority unless there is community consensus.