Manual clippy fixes

[lionel-]

Branched from #1108
Progress towards #1104

Main lints fixed:

• Too many arguments
• Unnecessary try_ conversions (can just use non-failing conversions)
• Large enum variants. Sometimes fixed by boxing, sometimes by disabling the lint.
• Use &str instead of &String, &Path instead of &PathBuf, &[] instead of &Vec
• Clippy doesn't like uppercase variants like EOF, so they've become e.g. EndOfFile
• Removed common prefixes/suffixes from enum variants. I added an opt out for the LSP notification enum so we can keep the Did prefix, e.g. KernelNotification::DidOpenVirtualDocument.
• into_ methods must take self. Renamed those that didn't, e.g. Dap::make_variables() which is not really an "into" method strictly speaking.
• Simplified unnecessary uses of match
• Use .is_some() or is_err() methods when applicable
• Use ? instead of explicit return when applicable
• Build regex in lazy globals

[lionel-]

These new structs fix the "too many arguments" lint.