Add support for placeholder-based artifacts

[mainnet-pat]

Placeholder-based artifacts are those which might be produced elsewhere or contain raw bitcoin script. Placeholders are used to insert the properly encoded variables at certain places in the script to produce a valid contract.

This pattern first emerged in our work for bch-xmr atomic swaps, now in another project where I had to support the EC-compatible 2 of 3 multisig contract.

Also add a sample 2 of 3 multisig contract and tests to highlight the feature and demonstrate the usage patterns.

[vercel]

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Name	Status	Preview	Comments	Updated (UTC)
cashscript	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Jul 14, 2025 2:30pm

[codecov]

Codecov Report

Attention: Patch coverage is 98.05825% with 2 lines in your changes missing coverage. Please review.

Project coverage is 97.41%. Comparing base (47a9dbc) to head (7428e37).
Report is 9 commits behind head on master.

Files with missing lines	Patch %	Lines
packages/cashscript/src/Argument.ts	96.07%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #321      +/-   ##
==========================================
+ Coverage   97.35%   97.41%   +0.06%     
==========================================
  Files          76       79       +3     
  Lines       17440    17743     +303     
  Branches     1274     1317      +43     
==========================================
+ Hits        16978    17285     +307     
+ Misses        455      447       -8     
- Partials        7       11       +4     

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