expose ShapeStream.fetchSnapshot as public api

[samwillis]

This exposes the ShapeStream.fetchSnapshot as a public api that can be used to fetch a snapshot without it being injected into the emitted stream of change messages. This is useful for cases where the user wants to handle the application of these snapshot in a custom way, as we need to in Tanstack DB for the progressive mode.

While working on this I also took the opportunity to tidy up the parsing of the messages, in the old implementation we would JSON.stringify the messages, only to the parse them again. I refactored the parser to be able to work on already parsed (JSON -> basic JS) messages.

First commit is the core change to expose fetchSnapshot, second commit is the refactor of message parsing.

[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 67.64%. Comparing base (6bb011b) to head (e1cf8b8).
⚠️ Report is 12 commits behind head on main.

Files with missing lines	Patch %	Lines
packages/typescript-client/src/parser.ts	82.35%	3 Missing ⚠️
packages/typescript-client/src/client.ts	88.88%	2 Missing ⚠️

Additional details and impacted files

@@ Coverage Diff @@## main #3463 +/- ##
==========================================
+ Coverage 67.45% 67.64% +0.18% 
==========================================
Files 180 180 Lines 9671 9693 +22 Branches 359 365 +6 ==========================================
+ Hits 6524 6557 +33 + Misses 3145 3134 -11 
Partials 2 2 

Flag	Coverage Δ
elixir	64.16% <ø> (+0.12%)	⬆️
elixir-client	73.94% <ø> (-0.53%)	⬇️
packages/experimental	88.95% <ø> (+1.22%)	⬆️
packages/react-hooks	86.48% <ø> (ø)
packages/typescript-client	93.60% <85.71%> (+0.03%)	⬆️
packages/y-electric	55.12% <ø> (ø)
postgres-140000	62.95% <ø> (+0.12%)	⬆️
postgres-150000	62.97% <ø> (?)
postgres-170000	62.71% <ø> (-0.08%)	⬇️
postgres-180000	62.89% <ø> (+0.25%)	⬆️
sync-service	63.17% <ø> (+0.19%)	⬆️
typescript	87.32% <85.71%> (+0.25%)	⬆️
unit-tests	67.64% <85.71%> (+0.18%)	⬆️

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commit: e1cf8b8