Reject {n} redirection prefix

[magicant]

This commit implements recognition of I/O location tokens that are always rejected as a syntax error. The shell may validly accept the I/O location syntax and semantics in the future.

Summary by Copilot

This pull request introduces a new reserved syntax for file descriptors in braces before redirection operators, updates the parser to handle this syntax as a syntax error, and includes corresponding documentation and test updates. Below are the most important changes grouped by theme:

Syntax Reservation and Parsing Updates:

• Added a new token type TT_IO_LOCATION to the parser to represent the reserved {name} syntax before redirection operators. Updated the next_token function to identify and classify this syntax as TT_IO_LOCATION. [1] [2]
• Modified the tryparse_redirect function to treat TT_IO_LOCATION tokens as syntax errors, with appropriate error messages.
• Updated alias substitution and operand validation logic to account for TT_IO_LOCATION tokens, ensuring they are rejected where appropriate. [1] [2]

Documentation Updates:

• Updated doc/redir.txt and doc/ja/redir.txt to document the reserved {name} syntax for future use, noting that it currently results in a syntax error. [1] [2]

News and Release Notes:

• Added entries to NEWS and NEWS.ja files to announce the reservation of the {name} syntax for file descriptors and its current treatment as a syntax error. [1] [2]

Test Coverage:

• Added new test cases in tests/redir-y.tst to verify that {name} syntax is correctly identified as unsupported and results in syntax errors.