Previously we changed the default version of the lambda-nodejs Function
construct to go from using the `builtInNodeJsCustomResourceRuntime`, a
map of regions to available versions, to `lambda.Runtime.NODEJS_18_X`.
The default `externalModule` configuration excluded the aws-sdk
version based on the runtime passed, excluding v2 for Node16 and under,
and v3 for Node18 and up, but users can pass their own bundling
configuration excluding `aws-sdk` while not explicitly passing a
runtime, which caused their functions to break.

Adds a new `lambda.Runtime` value for `NODEJS_LATEST`. This is central
reference for the latest version of NodeJS provided by the lamdba
service. It also includes a new property `isLatest` which can be used to
indicate that the runtime version may change over time. This can used to
indicate that relying on packages shipped with the environment may not
be relied upon if the version changes. We default to using the
`NODEJS_LATEST` runtime only if the feature flag is enabled. If the flag
is not enabled, use `NODEJS_16_X` to keep supporting users current
bundling configurations.

Additionally, add a warning to tell users if they are excluding a
package from their bundling that we know doesn't exist within the
runtime they are using. IE, if using `NODEJS_18_X` and the exclude list
includes `aws-sdk`, warn users that it won't be present.

fixes: #26732

Adds to the `externalModules` doc string to tell the default is `[]`
when `NODEJS_LATEST` runtime is used.

Adds to the README to detail that `externalModules` defaults to empty
when using `NODEJS_LATEST` and that `NODEJS_LATEST` is the default when
the feature flag is enabled. Also detail that `NODEJS_LATEST` is
designed to update as new versions are available and packages vended
with the environment may change.

Add a warning that is emitted when using `NODEJS_LATEST` and any
dependencies are external.