gie: implement a strict mode with <gie-strict> </gie-strict> (fixes #2158)

[rouault]

In that mode:

• All non-comment/decoration lines must start with a valid tag
• Commands split on several lines should be terminated with " \"

@busstoptaktik @kbevers Before I make the effort of converting all gie files in test/gie/ with those rules, I want to have your blessing

[busstoptaktik]

I have no strong feelings around this, although I probably would have preferred the syntax I proposed in #2158: <gie-strict> ... </gie-strict>, which more clearly indicates that it is the entire environment that is to be considered strict.

If I understand your proposed syntax correctly, the strictness may shift half way through the input file (in which case, I believe there should also be a way to switch back to lax mode again).

Anyway - most important to me is to keep the original syntax as the default, as it really is quite useful in its own right, as demonstrated in the PROJ teaching material, I linked to over in #2158.

[rouault]

<gie-strict> ... </gie-strict>

ah sorry I missed that part, and was actually thinking about something similar. Just pushed this

[busstoptaktik]

ah sorry I missed that part, and was actually thinking about something similar. Just pushed this

Groovy :-)

[rouault]

All .gie files converted and documentation added in gie.rst. There were a few instances of misspelled expect in builtins.gie

[rouault]

Just got feedback from @ccrook that he also got hit by expect/except misspelling, so I see a great future for <gie-strict> :-)