Keep only the beginning and the end of a long token for error messages

For "last read" in parse errors, the lexer keeps the text of the token it
reads. The token start moves only at strings and numbers, so everything
after a string or number belongs to it until the next one starts. Input
that cannot be read again copies that text while parsing (the istream and
FILE* adapters in their carry buffer, the lexer in token_string for
iterators), so valid input of 1 followed by 100,000,000 newlines peaked
at about 225 MB, and a long string was kept a second time.

Replace the carry buffer and token_string by token_text_buffer, which
keeps the first 4096 and at least the last 4112 characters of a token and
drops the characters in between where it would otherwise grow, so
appending stays as cheap as before. get_token_string() shows a token of
more than 8192 characters as its first and last 4096 characters around
"...", the same for every kind of input; contiguous input copies only
these parts.

The example input now peaks at 1.8 MB. Instruction counts of parsing
from std::string, std::istream and FILE* are unchanged (clang 21 and
GCC 15); parsing from std::list<char> needs 1 to 17 % fewer.

Fixes #6120.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/home/exceptions.md

@@ -140,6 +140,8 @@ Exceptions have ids 1xx.
 
 This error indicates a syntax error while deserializing a JSON text. The error message describes that an unexpected token (character) was encountered, and the member `byte` indicates the error position.
 
+The message ends with the last read token (`last read: '...'`), with control characters written as `<U+XXXX>`. A token of more than 8192 characters is shown by its first and its last 4096 characters, separated by `...`.
+
 !!! failure "Example message"
 
     Input ended prematurely: