Fix FunctionDeclaration spec: Id may be null

[ghost]

Q	A
Bug fix?	no
Breaking change?	no
New feature?	no
Deprecations?	no
Spec compliancy?	yes
Tests added/pass?	Tests pass
Fixed tickets	#502
License	MIT

FunctionDeclaration may have a null id in export default function() {}. Updated its spec and type declaration.

[hzoo]

I was going to go with

ExportDefaultDeclaration

interface OptFunctionDeclaration <: FunctionDeclaration {
    id: Identifier | null;
}

interface OptClassDeclaration <: ClassDeclaration {
    id: Identifier | null;
}

interface ExportDefaultDeclaration <: ModuleDeclaration {
    type: "ExportDefaultDeclaration";
    declaration: OptFunctionDeclaration | OptClassDeclaration | Expression;
}

like in estree/estree#98 (comment) if that makes sense?

[codecov]

Codecov Report

Merging #503 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #503   +/-   ##
=======================================
  Coverage   98.26%   98.26%           
=======================================
  Files          22       22           
  Lines        3524     3524           
  Branches      977      977           
=======================================
  Hits         3463     3463           
  Misses         22       22           
  Partials       39       39

Flag	Coverage Δ
#babel	80.98% <ø> (ø)	⬆️
#babylon	97.1% <ø> (ø)	⬆️

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