@Allowed decorator forces case sensitivity even though ARM doesn't require it

**Bicep version**
bicep --version
Bicep CLI version 0.3.255 (589f0375df)

**Describe the bug**
Adding an @allowed syntax decorator forces a parameter to be case sensitive even though the ARM parameter is not.

**To Reproduce**
A simple repo is a parameter that takes valid Azure regions

``` bicep
@allowed([
'EastUS'
'EastUS2'
'CentralUS'
'NorthCentralUS'
'SouthCentralUS'
'WestCentralUS'
'WestUS'
'WestUS2'
'CanadaEast'
'CanadaCentral'
'BrazilSouth'
'NorthEurope'
'WestEurope'
'FranceCentral'
'FranceSouth'
'UKWest'
'UKSouth'
'GermanyCentral'
'GermanyNortheast'
'GermanyNorth'
'GermanyWestCentral'
'SwitzerlandNorth'
'SwitzerlandWest'
'NorwayEast'
'NorwayWest'
'SoutheastAsia'
'EastAsia'
'AustraliaEast'
'AustraliaSoutheast'
'AustraliaCentral'
'AustraliaCentral2'
'ChinaEast'
'ChinaNorth'
'ChinaEast2'
'ChinaNorth2'
'CentralIndia'
'WestIndia'
'SouthIndia'
'JapanEast'
'JapanWest'
'KoreaCentral'
'KoreaSouth'
'SouthAfricaWest'
'SouthAfricaNorth'
'UAECentral'
'UAENorth'
])
param targetResourceRegion string
````

You cannot pass in "eastus" even though its a perfectly valid region

**Additional context**
I do realize this is a crappy way to code this, but this is directly from Microsft docs: https://docs.microsoft.com/en-us/azure/azure-monitor/alerts/resource-manager-alerts-metric#template-file-6

Possible solutions
- Make the @Allowed decorator case insensitive
- Add a decorator @CaseInsensitive
- Add decorator types @AzureRegion - which may not work (run time vs compile time when looking at different AzureEnvironments (i.e. Azure vs AzureUSGovernment)

Start by reproducing the issue with the Bicep CLI and the provided @allowed region parameter example, then compare its validation with ARM's case-insensitive behavior. Review the implementation and tests for the @allowed decorator; done means a value such as "eastus" is accepted while invalid regions remain rejected.

Written by the indexing model from the issue text.