Type annotation without `def` fails to parse in workflow body

Bug report

A variable declaration with a type annotation but no def keyword fails
to parse inside a workflow block — the parser interprets name: as a
section label and reports a misleading error.

For consistency with the other declaration forms (see matrix below), this
form should be accepted. Regardless of that decision, the error message
should be improved to point at the offending line and explain the actual
cause.

Reproduction

record Sample {
    id: String
    fastq_1: Path
    fastq_2: Path?
}

def hello(sample: Sample) {
    println "Hello sample ${sample.id}!"
}

workflow {
    sample1: Sample = record(id: '1', fastq_1: file('1_1.fastq'), fastq_2: file('1_2.fastq'))
    hello(sample1)
}

Error

Error script.nf:12:1: Invalid workflow definition -- check for missing or out-of-order section labels
 12 | workflow {
    | ^^^^^^^^^^
 13 |     sample1: Sample = record(...)

The error points at workflow { rather than the real offending line, and
the message ("out-of-order section labels") does not describe the actual
problem.

Matrix tested

Expected

• For consistency with the other three forms, the type-annotated form
  without def should be accepted.
• In any case, the error message should point at the offending line and
  clearly explain the cause (e.g. "a declaration keyword is required with
  a type annotation" if the restriction is intentional).

Start by reproducing the four declaration forms in the reported workflow body and compare their parser behavior and diagnostics. Trace how workflow declarations are parsed, then verify that the type-annotated form is either accepted or produces a line-specific message explaining the restriction.