Clean `rustc/parse/src/lexer` to improve maintainability
This PR refactors the lexer code to improve maintainability and eliminate code duplication.
In the first commit, I improve the error handling:
- rename `make_unclosed_delims_error` to more appropriate `make_mismatched_closing_delims_errors`
- changes return type from Option<Diag> to `Vec<Diag>` to avoid lengthy vec processing at `lex_token_trees`
- use `splice` instead of `extend` to make the logic clearer, since `errs` sounds more generic and better suited as a return value
In the second commit, I replace the magic number 5 with UNCLOSED_DELIMITER_SHOW_LIMIT constant.
In the third commit, I moves `eof_err` function below parsing logic for better code flow.
In the forth one, I extract `calculate_spacing` function to eliminate duplicate spacing logic between `bump` and `bump_minimal` functions.
r? compiler
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