Add LiteralRef wrapper to assign ref-like values without resolving

[philippjfr]

• Implements Add raw wrapper to assign ref-like values without resolving #1090
• Adds tests
• Add docs

[codecov]

Codecov Report

❌ Patch coverage is 92.85714% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 89.22%. Comparing base (f048bd3) to head (741f970).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
param/parameterized.py	92.85%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1091      +/-   ##
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+ Coverage   89.15%   89.22%   +0.07%     
==========================================
  Files           9        9              
  Lines        4685     4726      +41     
==========================================
+ Hits         4177     4217      +40     
- Misses        508      509       +1     

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[maximlt]

This looks like a useful functionality for advanced users and libraries that integrate Param (like Panel, Lumen, etc.). A few comments:

• I don't find raw or the other suggested names in the issue (constant, const, value) to be explicit enough and express that this is to wrap a reference so that it is not resolved eagerly by Param. How about Ref(<ref>, literal=True)? (I'm also fine with raw=True, or resolved=False). Or RefLiteral()?
• I'm not convinced we need a utility like raw in addition to the class Raw (or a ref utility in addition to the Ref class I suggested above). It made sense with rx but this feature is clearly a simple wrapper class, that is going to be used sparingly for advanced cases (unlike rx that is for all users and can be used a lot in a module).
• I'd prefer if this were not exposed in the top-level module, as it's a feature that I don't expect to be used that much.

[maximlt]

Somehow I missed until now that the wrapper was just transient. I think I stopped at "without resolving". I'm okay with that but was just wondering, @philippjfr is unwrapping more convenient than keeping it as-is? Unless you already know the Parameter is holding a literal ref, I guess you need a is_ref function? While if it's still wrapped as a LiteralRef, it's just an isinstance call?

[maximlt]

Suggested change

	Example
	-------
	Examples
	--------

[maximlt]

Just to avoid the possible confusion that this would only work with the base Parameter.

Suggested change

	" value = param.Parameter(allow_refs=True)\n",
	" value = param.String(allow_refs=True)\n",

[philippjfr]

Well this will error, because the ref value (i.e. the Parameter instance) is not a string.

[maximlt]

Suggested change

	"#### Key points\n",
	"### Key points\n",

[maximlt]

Suggested change

	"#### When to use LiteralRef\n",
	"### When to use LiteralRef\n",

[jbednar]

LiteralRef sounds fine, but is it guaranteed to be a reference? Isn't it just a lazy, unevaluated object in general? The concept seems to be the same as LISP's quote operator.

[maximlt]

Can you also add parameterized.LiteralRef to this file https://github.com/holoviz/param/blob/main/doc/reference/param/parameterized.md#helpers so that it's documented in the API?