Implicit enum values after explicit numeric variants duplicate the explicit value

[aspizu]

Repro

tmp=$(mktemp -d /tmp/goboscript-enum-mixed-XXXXXX)
printf 'costumes "blank.svg";\n\n' > "$tmp/stage.gs"
printf 'costumes "blank.svg";\n\nenum Direction {\n    A = "A",\n    B,\n    C = "C",\n    D,\n    E = 2,\n    F,\n}\n\nonflag {\n    say Direction.B;\n    say Direction.D;\n    say Direction.F;\n}\n' > "$tmp/main.gs"
printf '<svg xmlns="http://www.w3.org/2000/svg" width="1" height="1"></svg>\n' > "$tmp/blank.svg"
cargo run --quiet -- build "$tmp"
python3 - <<'PY' "$tmp/$(basename "$tmp").sb3"
import json, sys, zipfile
with zipfile.ZipFile(sys.argv[1]) as z:
    p=json.loads(z.read('project.json'))
blocks=p['targets'][1]['blocks']
for b in blocks.values():
    if b.get('opcode') == 'looks_say':
        print(b['inputs']['MESSAGE'])
PY

Observed message constants:

[1, [4, 0]]
[1, [4, 1]]
[1, [4, 2]]

So Direction.F compiles to 2 even though Direction.E is also explicitly 2.

Docs in docs/language/enums.md show this exact mixed enum pattern and mark F as 3.

Expected

After an explicit numeric enum value, the next implicit enum value should continue at the following number. Here Direction.F should compile to 3.

[linear]

GS-275