Support float values

Follow up from #486

We need to add support for floating point values that can be entered by user.
Floating point values - numbers with dot i.e. `3.14` have following constraints:

* Float values can be entered manually only in the code as a variable or object value
* They cannot be a parameter
* They cannot be an output of an function
* They are not allowed as part of expressions
* There's `float()` function that should be supported - it can be used only in variables and as object values

Valid usage in ARM:
```
{
"$schema": "https://schema.management.azure.com/schemas/2019-04-01/deploymentTemplate.json#",
"contentVersion": "1.0.0.0",
"parameters": {
"inPi": {
"type": "string",
"defaultValue": "3.14"
}
},
"functions": [],
"variables": {
"pi": 3.14,
"objectPi": {
"pi": 3.14
},
"pi2": "[float('3.14')]",
"pi3": "[float(variables('pi'))]",
"pi4": "[float(parameters('inPi'))]"
},
"resources": [],
"outputs": {
"out0": {
"type": "object",
"value": "[createObject('pi', variables('pi'))]"
},
"out1": {
"type": "object",
"value": "[variables('objectPi')]"
},
"out2": {
"type": "object",
"value": "[createObject('pi2',variables('pi2'))]"
},
"out3": {
"type": "object",
"value": "[createObject('pi3',variables('pi3'))]"
},
"out4": {
"type": "object",
"value": "[createObject('pi4',variables('pi4'))]"
}
}
}
```

Start with follow-up issue #486 and review the valid ARM template example in this issue. Trace how Bicep handles numeric values, variables, parameters, expressions, outputs, and the float() function, then verify that the listed allowed and disallowed usages are covered by tests.

Written by the indexing model from the issue text.