Correctly determine the category code of the ampersand (@) during lexical analysis

Make the following changes:

Add a new string option make_at_letter that specifies the category code of the ampersand (@) to explcheck-config.lua.
- The possible values are true, false, and "auto".
- The default value is "auto".
Add a corresponding new command-line option --make-at-letter to explcheck-cli.lua.
If the option is enabled, consider @ a letter during lexical analysis. Otherwise, consider @ an "other" character.
Use context clues to determine whether the option should be enabled or disabled when the value is "auto".
- Initially, the presence of LaTeX2e commands from LaTeX for package and class authors or LaTeX3 commands like \ProvidesExplClass and \ProvidesExplPackage is a strong indicator that the option should be enabled.
- In the future, we may want to analyze the use of the LaTeX2e commands \makeatletter and \makeatletter to determine for which parts of a file the option should be enabled.

As discussed in #30.

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