Warn on non-camelCase naming

[iwahbe]

Fixes #355

Generated warnings look like this:

name: yaml
runtime: yAML
Resources:
  provider:
    type: pulumi:providers:aws
    PrOperties:
      region: us-east-1
    DefaultProvider: true
  bucket:
    Type: aws:s3:BucketV2

Diagnostics:
  pulumi:pulumi:Stack (yaml-dev):
    Warning: 'Resources' looks like a miscapitalization of 'resources'
      on Pulumi.yaml line 3:
       3: Resources:
    Pulumi YAML will enforce camelCase capitalization by GA. See https://github.com/pulumi/pulumi-yaml/issues/355 for details.
    Warning: 'PrOperties' looks like a miscapitalization of 'properties'
      on Pulumi.yaml line 6:
       6:     PrOperties:
    Pulumi YAML will enforce camelCase capitalization by GA. See https://github.com/pulumi/pulumi-yaml/issues/355 for details.
    Warning: 'DefaultProvider' looks like a miscapitalization of 'defaultProvider'
      on Pulumi.yaml line 8:
       8:     DefaultProvider: true
    Pulumi YAML will enforce camelCase capitalization by GA. See https://github.com/pulumi/pulumi-yaml/issues/355 for details.
    Warning: 'Type' looks like a miscapitalization of 'type'
      on Pulumi.yaml line 10:
      10:     Type: aws:s3:BucketV2
    Pulumi YAML will enforce camelCase capitalization by GA. See https://github.com/pulumi/pulumi-yaml/issues/355 for details.

[AaronFriel]

LGTM, left a nit on the warning message.