👌 Add ability to localize figure caption

[j123b567]

Sphinx localization needs node source and line and if it is not present, localization step is skipped.

If the source looks like (MyST markdown)

::::{subfigure} A

:::{image} img1.png
:alt: Subfigure caption
:width: 100px
:::

Figure caption
::::

Then after running make gettext, this .pot file is generated

#: ../../index.md:1
msgid "Subfigure caption"
msgstr ""

After applying the PR, this .pot file is generated

#: ../../index.md:1
msgid "Subfigure caption"
msgstr ""

#: ../../index.md:8
msgid "Figure caption"
msgstr ""

[codecov-commenter]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (fb52892) 80.65% compared to head (c79156f) 79.78%.

Files	Patch %	Lines
src/sphinx_subfigure/main.py	0.00%	4 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #14 +/- ## ========================================== - Coverage 80.65% 79.78% -0.88% ========================================== Files 5 5 Lines 274 277 +3 ========================================== Hits 221 221 - Misses 53 56 +3 ```

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