Support 'quotedTemplate' in addition to 'StrLit' in block labels.

[semtexzv]

Currently, block label can be either identifier or string literal. I kind of want to support following pattern in a DSL I'm working on

template {
    repeat = 10
    vars {
        prefix = "Hello"
    }
    generate {
        object "${vars.prefix}-${repeat}" {
            
        }
    }
}

But the HCL spec explicitly only allows strings or literals as object keys.

Lifting the quotedTemplate from just subpattern of TemplateExpr into a rule that could be used as block label would greatly extend the usability, and reduce the need for patterns like for_each in terraform.

TL;DR: In Syntax spec:
Replace

Block        = Identifier (StringLit|Identifier)* "{" Newline Body "}" Newline;

with:

Block        = Identifier (StringLit|Identifier|QuotedTemplate)* "{" Newline Body "}" Newline;