Correctly export the main fn in the json abi

[xunilrj]

Description

This fixes a bug introduced by #5427, where the ABI was exporting the "__entry" function instead of the "main".

When parsing and type checking I now mark function as "main" an only later decide if the compilation will call the main or the entry.

I also moved the is_entrypoint method from the TyAstNode to the DCE pass, because it is super specific to that pass.

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.

[github-actions]

Benchmark for fa4f096

Click to view benchmark

Test	Base	PR	%
code_action	5.4±0.04ms	5.4±0.18ms	0.00%
code_lens	292.4±25.80ns	289.3±13.15ns	-1.06%
compile	5.9±0.11s	5.9±0.05s	0.00%
completion	5.0±0.20ms	4.9±0.08ms	-2.00%
did_change_with_caching	5.4±0.07s	5.3±0.04s	-1.85%
document_symbol	979.2±20.51µs	1033.5±17.53µs	+5.55%
format	70.4±1.59ms	70.9±2.25ms	+0.71%
goto_definition	358.3±4.76µs	355.7±5.74µs	-0.73%
highlight	9.1±0.15ms	9.1±0.11ms	0.00%
hover	592.1±6.47µs	581.7±7.77µs	-1.76%
idents_at_position	122.8±0.51µs	122.6±1.05µs	-0.16%
inlay_hints	666.8±22.31µs	677.2±27.62µs	+1.56%
on_enter	503.4±17.65ns	487.9±14.70ns	-3.08%
parent_decl_at_position	3.7±0.03ms	3.8±0.06ms	+2.70%
prepare_rename	372.8±11.86µs	358.0±5.69µs	-3.97%
rename	9.6±0.06ms	9.5±0.12ms	-1.04%
semantic_tokens	1055.9±6.67µs	1060.5±35.13µs	+0.44%
token_at_position	364.5±2.55µs	355.6±4.77µs	-2.44%
tokens_at_position	3.7±0.05ms	3.7±0.04ms	0.00%
tokens_for_file	416.3±2.76µs	416.7±5.53µs	+0.10%
traverse	44.6±1.52ms	44.1±1.22ms	-1.12%