[spec] grammar blocks use unspecified notation

[dlangBugzillaToGithub]

Dennis (@dkorpel) reported this on 2021-11-25T12:22:33Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=22543

Description

Newsgroup discussion:
https://forum.dlang.org/post/gwcvsqctgsuetdyfenyz@forum.dlang.org

Blocks like:
```
SpecialTokenSequence:
    # line IntegerLiteral Filespecopt EndOfLine
    # line __LINE__ Filespecopt EndOfLine
```

```
ByteOrderMark:
    \uFEFF

Shebang:
    #! Charactersopt EndOfShebang

EndOfShebang:
    \u000A
    EndOfFile
```

Have a certain meaning:
- an identifier with a colon denotes a rule
- the following lines denote alternatives
- spaces between elements of a rule denote either immediate concatenation (lexical grammar), or optional separation by Comments and WhiteSpace (parsing grammar)
- the 'opt' suffix means 'optional'
- text in boldface denotes a string / character literal (lexical grammar) or single token (parsing grammar)

But none of this is specified.