ui: add source and assembly view for callstack sample functions

Add a details tab showing a function's source and disassembly, each
line and instruction annotated with the number of samples taken on it
(self) and including its callees (total), shaded by share of the
hottest row. The data comes from the source_file and disassembly tables
populated by `trace_processor bundle` and the callstacks.annotate
stdlib module; when neither source nor disassembly was bundled the tab
explains how to bundle them.

Source is syntax highlighted for C, C++ and Rust and instructions for
their mnemonics, registers and numbers, with a small dependency-free
tokenizer. The assembly view draws branches to targets within the
function as arrows in a gutter, packed into lanes with loops coloured
distinctly, each row rendering its own slice so the listing stays
virtualized.

The tab opens from a "View source & assembly" action on flamegraph
nodes of the stack sample flamegraphs, both for area selections and for
single samples. To locate the function, the stack sample metrics carry
the source file, sampled address and mapping id of each node as hidden
properties.