Fixed width areas not allowing tab after leading colon.

[Tom Alexander]

The documentation for fixed width areas states: A “fixed-width line” starts 
with a colon character (:) and either a whitespace character or the immediate 
end of the line. 

Using the test document:
```
:<TAB>foo
```

parses as a paragraph instead of a fixed-width area:
```
(org-data
(:standard-properties
  [1 1 1 7 7 0 nil org-data nil nil nil 3 7 nil #<buffer *scratch*> nil nil nil]
  :path nil :CATEGORY nil)
(section
  (:standard-properties
   [1 1 1 7 7 0 nil first-section nil nil nil 1 7 nil #<buffer *scratch*> nil 
nil #0])
  (paragraph
   (:standard-properties
    [1 1 1 7 7 0 nil top-comment nil nil nil nil nil nil #<buffer *scratch*> 
nil nil #1])
   #(": foo\n" 0 6
     (:parent #2)))))
```

This happens in a document in worg: 
https://git.sr.ht/~bzg/worg/tree/74e80b0f7600801b1d1594542602394c085cc2f9/item/org-contrib/org-bom.org#L499

Emacs version: GNU Emacs 29.1 (build 1, x86_64-pc-linux-musl)
Org-mode version: c703541ffcc14965e3567f928de1683a1c1e33f6 (latest in git)

Fixed-width area documentation: 
https://orgmode.org/worg/org-syntax.html#Fixed_Width_Areas
--
Tom Alexander