Avoid parsing snippets with placeholders

[anthony-c-martin]

On my other branch, I was running into a huge number of ExpectedTokenException errors on startup, because the SnippetProvider processes the snippets after placeholder replacement - at which point in time, the file is not a syntactically valid Bicep file. This is unnecessary, and in my branch actually resulted in invalid snippets being generated due to syntax ambiguity.

Here I've modified SnippetProvider to do its analysis on the snippets with placeholders instead (which are validated for parse errors on checkin).

To give a concrete example, previously we were trying to parse the following (note that ${ and } are not recognized as valid Bicep syntax:

resource ${1:aksCluster} 'Microsoft.ContainerService/managedClusters@2021-03-01' = {
  name: ${2:'name'}
  location: ${3:location}
...

Now we're trying to parse the following:

resource /*${1:aksCluster}*/aksCluster 'Microsoft.ContainerService/managedClusters@2021-03-01' = {
  name: /*${2:'name'}*/'name'
  location: /*${3:location}*/'location'
...

[bhsubra]

nit: formatting is off.

[bhsubra]

Are these applicable to changes in this PR?