Release Dash v2.4.0 · plotly/dash

#1952 Improved callback_context
- Closes #1818 Closes #1054
- adds dash.ctx, a more concise name for dash.callback_context
- adds ctx.triggered_prop_ids, a dictionary of the component ids and props that triggered the callback.
- adds ctx.triggered_id, the id of the component that triggered the callback.
- adds ctx.args_grouping, a dict of the inputs used with flexible callback signatures.
#2009 Add support for Promises within Client-side callbacks as requested in #1364.
#1956 Add TypeScript components generation.
#2034 Add link_target prop to dcc.Markdown component. Closes #1827
#2035 Add type annotations to testing fixtures.

#2029 Restrict the number of props listed explicitly in generated component constructors - default is 250. This prevents exceeding the Python 3.6 limit of 255 arguments. The omitted props are still in the docstring and can still be provided the same as before, they just won't appear in the signature so autocompletion may be affected.
#1968 Fix bug #1877, code which uses merge_duplicate_headers and style_header_conditional to highlight columns, it incorrectly highlights header cells.
#2015 Fix bug #1854 in which the combination of row_selectable="single or multi" and filter_action="native" caused the JS error.
#1976 Fix #1962 in which DatePickerSingle and DatePickerRange are extremely slow when provided a long list of disabled_days.
#2035 Fix #2033 In-App error reporting does not render HTML.
#1970 dcc.Dropdown Refactor fixes:
- Fix bug #1868 value does not update when selected option removed from options.
- Fix bug #1908 Selected options not showing when the value contains a comma.

#1751 Rename app.run_server to app.run while preserving app.run_server for backwards compatibility.
#1839 The callback decorator returns the original function, not the wrapped function, so that you can still call these functions directly, for example in tests. Note that in this case there will be no callback context so not all callbacks can be tested this way.
#2016 Drop the 375px width from default percy_snapshot calls, keep only 1280px
#2027 Improve the error message when a user doesn't wrap children in a list

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