SPDX License Identifiers are a more compact / standardized
way of representing file license information.
See: https://spdx.dev/resources/use/#identifiers
This was done with the `ambr` search and replace tool.
ambr --no-parent-ignore --key-from-file --rep-from-file key.txt rep.txt *
In LibDebug this required implementing the Kill debug action, and
in HackStudio this required making the toolbar's stop action stop
the debugger if active.
HackStudio's debugger instance has its source root property updated
when switching projects, and breakpoints will properly canonicalize
their file paths as the Debugger now expects.
