Feature: String templates

[timotheeguerin]

fix #1591

String templates

Playground examples

import "@typespec/json-schema";

using TypeSpec.JsonSchema;

@jsonSchema
namespace Schemas;

alias myconst = "foobar";

model Person {
  simple: "Simple ${123} end";
  multiline: """
  Multi 
     ${123} 
    ${true}
  line
  """;

  ref: "Ref this alias ${myconst} end";
  template: Template<"custom">;
}

alias Template<T extends valueof string> = "Foo ${T} bar";

Other fixes

Also fixes https://github.com/Azure/typespec-azure/issues/3399(Show invalid escape sequence char instead of the whole string)

[github-actions]

Changes in this PR will be published to the following url to try(check status of TypeSpec Pull Request Try It pipeline for publish status):
Playground: https://cadlplayground.z22.web.core.windows.net/prs/2630/

Website: https://tspwebsitepr.z22.web.core.windows.net/prs/2630/

[timotheeguerin]

Do the names look good? Alternatives could have been

• TemplateString
• TemplateStringLiteral
• TemplateLiteral
• StringTemplateLiteral

[daviwil]

StringTemplateLiteral is probably more accurate

EDIT: Ahh, I didn't see the rest of the names. Probably don't want all of them to be StringTemplateLiteralX so StringTemplate is fine IMO

[daviwil]

Do you split this into head/middle/end so that you only capture the part that needs to be computed as the middle?

[timotheeguerin]

Those are technically slightly different nodes and that's how typescript also structure their template so thought be good to keep inline as they must have had a reason to do that. But right now the data inside is basically the same

[bterlson]

I think this grammar is incomplete because we cannot reach StringTemplateMiddle/StringTemplateTail from the top-level goal symbol InputElement. We will need a separate goal symbol (say, InputElementTemplateTail). I also think it should be a parse error to have an empty substitution, which would require separating out right brace from the list of tokens and only allowing it with the InputElementTemplateTail.

[timotheeguerin]

updated, let me know if that looks better