Breaking changes for improved startup time and error handling!

Thanks to @Enselic for basically all the work to make this release happen!

Breaking changes

• Lazy-load syntaxes to significantly improve startup time. This changes the binary format of syntax dump files.
• Remove ContextId::new() from public API to support lazy-loading of syntaxes
• Rename HighlightLines::highlight() to HighlightLines::highlight_line() to make it clear that the function takes one line at a time
• Make plist dependency (used for loading themes) optional via new plist-load feature
• Remove obsolete dump-load-rs and dump-create-rs features that has been identical to dump-load and dump-create for two years
• Remove deprecated items ThemeSettings::highlight_foreground, ThemeSettings::selection_background, ClassedHTMLGenerator::new, ClassedHTMLGenerator::parse_html_for_line, html::css_for_theme, html::tokens_to_classed_html and html::tokens_to_classed_spans
• Mark all error enums as #[non_exhaustive]
• These functions have been changed to return a Result to allow propagation of errors:
  • html::ClassedHTMLGenerator::parse_html_for_line_which_includes_newline
  • html::append_highlighted_html_for_styled_line
  • html::css_for_theme_with_class_style
  • html::highlighted_html_for_string
  • html::line_tokens_to_classed_spans
  • html::styled_line_to_highlighted_html
  • parsing::ParseState::parse_line
  • parsing::ScopeStack::apply
  • parsing::ScopeStack::apply_with_hook
  • parsing::syntax_definition::Context::match_at
  • parsing::syntax_definition::ContextReference::id
  • parsing::syntax_definition::ContextReference::resolve

Other changes

• Fall back to Plain Text if a referenced syntax is missing
• Add support for hidden_file_extensions key in syntaxes.
• Implement Error and Display for all error enums by using thiserror
• Replace lazycell with once_cell to fix crash on lazy initialization
• Add ScopeRangeIterator
• Add CI check for Minimum Supported Rust Version. This is currently Rust 1.53.
• Make looking up a syntax by extension use case-insensitive comparison
• Make from_dump_file() ~15% faster
• Blend alpha value on converting colors to ANSI color sequences
• Fix sample code in documentation to avoid double newlines
• Fix lots of build warnings and lints
• Add Criterion benchmarks for a whole syntect pipeline and for from_dump_file()