Handle line continuation in Lexer::peek_char

[magicant]

The existing lexer implementation requires that the Lexer::line_continuations function be called every time before peeking or skipping a character where line continuation is allowed. This is error-prone because it can only covered by tests if we forget to call the function.

This PR modifies the lexer to automatically skip line continuation before peeking so that we no longer need to call the function, except in a few contexts where line continuation should not be recognized, i.e., single quotes, literal here-document contents, and comments.

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• Extract LexerCore
• Skip line continuation in Lexer::peek_char
• Remove all calls to Lexer::line_continuations
• Remove unnecessary tests about line continuation
• Remove unnecessary mentions to line continuation in doc comments