Consolidate `TablesNextToCode` and `GhcUnreigsterised` in configure (#15548)

TablesNextToCode is now a substituted by configure, where it has the correct defaults and error handling. Nowhere else needs to duplicate that, though we may want the compiler to to guard against bogus settings files.

I renamed it from GhcEnableTablesNextToCode to TablesNextToCode to:

• Help me guard against any unfixed usages

• Remove any lingering connotation that this flag needs to be combined with GhcUnreigsterised.

I split this off from !152 (merged) as I've had trouble with that one and CI. Trying to rule out of some stuff.

• are either individually buildable or squashed
• have commit messages which describe what they do (referring to Notes and tickets using #NNNN syntax when appropriate)
• have added source comments describing your change. For larger changes you likely should add a Note and cross-reference it from the relevant places.
• add a testcase to the testsuite.
• replace this message with a description motivating your change

If you have any questions don't hesitate to open your merge request and inquire in a comment. If your patch isn't quite done yet please do add prefix your MR title with WIP:.