Deprecate the keyword `var` on behalf of a new keyword `const`

**Is your feature request related to a problem? Please describe.**
The keyword `var` presumes the value assigned to the symbol is variable, that is, the concrete value referenced by the symbol changes over time. This is not the case with Bicep `var` symbols. A better label for them is `const` because their value remains unchanged once assigned.

Unlike other languages, Bicep doesn't support a mix mutable & immutable variables, so the distinction isn't important semantically for the compiler, but it does introduce ambiguity for people onboarding into Bicep

**Describe the solution you'd like**
A clear and concise description of what you want to happen.
Have `var` transition to `const`

The issue names no files, tests, or entry points. Start by locating how the Bicep compiler handles the `var` keyword and identify the compatibility, diagnostic, and migration scope for `const`. Done means the deprecation behavior and its impact on existing Bicep programs are defined and verified.

Written by the indexing model from the issue text.