The purpose of the path normalizer is to normalize all inbound paths
by making them lower case, and removing any "-" characters. Normalizing
paths means sources with different idiomatic approaches to defining
key values will all map correctly to the defined config classes.

The only downside to this is multiple config attributes in the same
class that differ only by case can no longer be disambiguated. This
should be a rare case and the advantages are more than worth losing
this "feature".

We also add a LowercaseParameterMapper by default which can handle
the normalized paths.

With path normalization (and even before path normalization), it was
possible for the reported path to not match the input value. For
example, with environment variables, the environment variable may have
been `FOO__BAR`, but the report showed `FOO.BAR`, which makes it harder
for users to trace the report back to the original source.

We now add a `sourceKey` attribute to `Node` and report on it, to show
user's the key value as it was originally present in the source.

Remove both the `SnakeCaseParamMapper` and `KebabCaseParamMapper`
by default, add the `PathNormalizer` by default.

Add removal of `_` to path normalizer.

Fix some issues with the `HikariDataSourceDecoder` when enabling
path normalization by default -- that decoder requires the
original key case as its props are case-sensitive. Create an
abstract `UnnormalizedKeysDecoder` which has the ability to restore
the case of keys via the `sourceKey`.

Fix breaking explicit sealed types with normalization because the
discriminator field defaults to `_type` which normalizes to `type`.
Disable normalization if the field name matches the discriminator field
name, and the node is a `MapNode`.

Fix reporting for strict mode to use the `sourceKey` value, so that
reporting matches the source value, not the normalized value.

Update Preprocessor implementations to correctly copy the source key
when new Map and Array nodes are created.

Resolves sksamuel#410.