Create a stack state for managing diagnostics. This will eventually
replace some of the global state used for diagnostics, as well as
providing a mechanism to add additional diagnostics.
For now, this is only the stack manipulation with a bare minimum of
interface logic (in particular, the `block` and `include` commands).
Nothing actually looks at the diagnostic state yet, and the primary user
interface does not yet exist (although there are some references to it
already).
Revise docs for all "Scripting Commands", except four find_XXX
that use a macro suite of their own.
* Take full advantage of the improved syntax highlighting.
* Make consequential use of <..> placeholders.
* Clarify things here and there in the text.
Specific improvements to some command docs:
* "math": Correct description of novel hexadecimal capability.
* "if", "foreach", "while": Provide link to "endif" etc
* "foreach", "while": Mention "break" and "continue".
* "foreach": Simplify explanation of ``RANGE`` and ``IN`` signatures;
advise against negative arguments or reverse ranges (compare issue #18461)
* "endif", "endfunction" etc: Explain that the argument is optional and
maintained for compatibility only
