(TODO: Read through the issues backlog and find issues this is connected to and put the reference number here so this will close them. There's possibly more than one such issue.)

This implements a compile-time check that makes it illegal for a script (or the interpreter) to create an identifier that clashes with any of the following:

• The built-in variable names (SHIP, NORTH, ALTITUDE, VELOCITY, etc)
• The system locks (THROTTLE, STEERING, etc)
• The built-in function names (PRINT(), HEADING(), V(), R(), etc)

Note that this is a compile-time check, so it detects the problem and complains even if the offending line of script code is never reached and executed.

The need for this PR is this:

Once one of these built-in identifiers gets clobbered by the script, it often cannot be accessible again for the rest of that program. (or the entire interpreter context).

The danger with this PR is this:

Because this was never enforced before, There's probably lots of scripts out there that may have accidentally used a name that masked a built-in, like the example below:

// R() is the built-in rotation constructor, and this next line
// replaces it with a new user-defined R() "function":
lock r to 1.

// But if the script never tries to use R() from here on,
// it might have worked anyway, blissfully unaware that it wiped out
// a built-in function.

This enforcement in this PR, while better practice, would break existing scripts that have things like the situation above, and require having them rename such variables. If it's a script the user is actively working on, that's no big deal. If it's a script the user found out on the internet, it is.

Thus there's an option to turn the old behavior back on again:
There's going to be two ways to toggle whether this rule is being enforced:

The default global behavior comes from: CONFIG:CLOBBERBUILTINS

There will also be a way to override it per-file with the compiler directive @CLOBBERBUILTINS on., which isn't implemented yet in the current state of this PR.

[x] Add @CLOBBERBUILTIN directive, which requires some
grammar file changes which is why I didn't tackle it yet.

[x] Add user-facing documentation about this, which means
mentioning it in several place through the docs, and
mentioning how older scripts you find on the internet
might break because of it unless you change the setting,
etc, etc, etc.