Implement format strings.

[ta0kira]

For example, Python has f"foo is: {foo}". Something like this would be easy to implement; it could just be translated into a String.builder() call. I think the biggest decision would be what syntax to use.

[ta0kira]

Some requirements:

1. Any expression should be allowed within a substitution block. This means that symmetric delimiters for substitution blocks that can also be used in expressions (e.g., `, |) must not be used.
2. The open delimiter for a substitution block should be escapable within the string, but should also be uncommon in strings. No need to make it multi-character (e.g., #{foo} in Ruby) if the start can just be escaped if needed.
3. Indicating a format string by prefixing the opening " should also be usable for raw strings later on if added, and shouldn't seem "weird" given other uses of the prefix. For example, probably not \"..." or @"...".

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Some possibilities:

1. $"foo is: {foo}", $"Bar is: { Bar{ 123 } }"
2. #"foo is: {foo}", #"Bar is: { Bar{ 123 } }"
3. %"foo is: {foo}", %"Bar is: { Bar{ 123 } }"

% might actually be the best option, since it's used in C format strings, and it isn't already being parsed as a unary operator. Note that %"foo is: %{foo}" might be confusing though, because it seems like %{foo} could refer to a param passed to the formatter.