fix: improve extension! macro

[magurotuna]

This commit makes the following refinements for extension! macro:

• Improve the doc comments to cover all the fields available now (i.e. external_references and customizer)
• Fix the wrong field name in the doc (config -> options)
• Fix the compile error regarding a wrong expansion of repetitive external_references elements
• Let it correctly handle the cases where options are provided but state_fn is not. For example, the previous implementation emits a compile error for the code below, which is fixed in this patch:

deno_core::extension!(
  my_ext,
  options = { value: String },
);

error: expected expression, found `;`
  --> src/bin/deno_core_extension.rs:15:1
   |
15 | / deno_core::extension!(
16 | |     my_ext,
17 | |     options = { value: String },
18 | | );
   | |_^ expected expression
   |
   = note: this error originates in the macro `$crate::extension` which comes from the expansion of the macro `deno_core::extension` (in Nightly builds, run with -Z macro-backtrace for more info)

[CLAassistant]

All committers have signed the CLA.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.90%. Comparing base (0c7f83e) to head (4e10dc5).
Report is 111 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #881      +/-   ##
==========================================
+ Coverage   81.43%   81.90%   +0.46%     
==========================================
  Files          97       98       +1     
  Lines       23877    24930    +1053     
==========================================
+ Hits        19445    20418     +973     
- Misses       4432     4512      +80     

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