Add partial render support using "template#block" syntax (fixes gin-gonic/gin#3745)

[YamiOdymel]

Description

As HTMX becomes more popular, a page should be able to decide whether to return a full response or a partial one that updates only part of the view.

Current Problem

The current multitemplate relies on Gin’s c.HTML(code int, name string, obj any) signature, which only allows rendering a whole "template set", not a specific template/block within "the set".

In htmx ~ Template Fragments, a template#block syntax is mentioned. This seems to be a fairly standard convention in template systems.

Implementation

The change is very simple: it just splits template name from hello#world and passes world to Gin’s render.HTML.Name, basically reusing what we already have.

c.HTML(http.StatusOK, "hello#world", gin.H{
	"fruit": "apple",
})

This makes it possible to render partials which is defined as {{ block "world" . }} and {{ define "world" }} within a template set.

---

related: [gin-gonic/gin] add partial rendering to context.HTML() or add context.HTMLBlock()

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Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 69.36%. Comparing base (32b2334) to head (296cfd9). :warning: Report is 28 commits behind head on master. :exclamation: Your organization needs to install the Codecov GitHub app to enable full functionality.

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HEAD has 12 uploads less than BASE

Flag	BASE (32b2334)	HEAD (296cfd9)
ubuntu-latest	7	3
go-1.14	1	0
go-1.15	1	0
go-1.16	1	0
go-	1	0
go-1.17	1	0
go-1.18	1	0
go-1.19	1	0
go-1.20	1	0

Additional details and impacted files

@@             Coverage Diff              @@
##            master      #77       +/-   ##
============================================
- Coverage   100.00%   69.36%   -30.64%     
============================================
  Files            2        2               
  Lines          112      235      +123     
============================================
+ Hits           112      163       +51     
- Misses           0       71       +71     
- Partials         0        1        +1     

Flag	Coverage Δ
go-	?
go-1.14	?
go-1.15	?
go-1.16	?
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go-1.18	?
go-1.19	?
go-1.20	?
go-1.23	69.36% <100.00%> (?)
go-1.24	69.36% <100.00%> (?)
go-1.25	69.36% <100.00%> (?)
macos-latest	69.36% <100.00%> (-30.64%)	:arrow_down:
ubuntu-latest	69.36% <100.00%> (-30.64%)	:arrow_down:

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[AxelRHD]

Is this something which is planned to be implemented in the near future? I will start an new project soon and it would save me a lot of extra code and files for the html-templating.

Thanks.