Expand `$LINENO` to a correct value in the prompt string

[magicant]

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In the current implementation, the $LINENO variable contained in the prompt string ($PS1 and $PS2) always expands to 1. This is because the expansion of the $LINENO parameter occurs in the first line of the prompt string variable value, which is parsed as text.

A more intuitive behavior would be to expand $LINENO to the line number in the current lexer context, but the current architecture does not allow this because we now create a separate lexer to parse the text.