Add default initialization for EOF token to auto-generate SourceFile convenience initializer

[fwcd]

The EOF token currently does not yield a .default_initialization() since it is not part of the SYNTAX_TOKEN_MAP (.token() therefore evaluates to None). This makes it slightly inconvenient to initialize SourceFile, since the eof argument has to be passed explicitly:

public init(
  eofToken: TokenSyntax,
  @CodeBlockItemListBuilder statementsBuilder: () -> ExpressibleAsCodeBlockItemList = { CodeBlockItemList([]) }
)

While the convenience initializer from SourceFileConvenienceInitializers.swift currently works around this, this PR makes the explicit convenience initializer superfluous, since the generated initializer is now essentially equivalent:

public init(
  eofToken: TokenSyntax = TokenSyntax.eof,
  @CodeBlockItemListBuilder statementsBuilder: () -> ExpressibleAsCodeBlockItemList = { CodeBlockItemList([]) }
)

cc @ahoppen

[kimdv]

@swift-ci please test

[ahoppen]

LGTM 👍