Recognize .jsonc files without implying JSON5 support

[leoshone]

Following the review comment on #251 ("Let's remove JSON5 as it is not supported currently by the plugin") - this is a version that does not accept the JSON5 language at all. It only recognises the .jsonc extension, and a real .json5 file keeps behaving exactly as before this change.

Why anything at all

Notepad++ maps the .jsonc extension to the json5 language ("json5 jsonc" share one entry in the default langs.xml), so IsJsonFile() - the gate for draw-on-open, follow-tab and auto-format - returned false for every jsonc document, even though the parser itself handles jsonc content fine (ignoring comments and trailing commas is configurable and on by default).

The change

IsJsonFile() now accepts the json5 language only when the current file's extension is .jsonc:

if (languageType == LangType::L_JSON)
    return true;

if (languageType == LangType::L_JSON5)
{
    auto fileName = GetCurrentFileName();
    StringHelper::ToLower(fileName);
    return fileName.ends_with(L".jsonc");
}

return false;

A genuine .json5 document (unquoted keys, single-quoted strings) does not match the extension check and keeps being ignored by the plugin exactly as it was before - no parsing, no error dialog, no change in behaviour.

Verification

Measured end to end in a real Notepad++ (8.9.6.2, default langs.xml):

• .jsonc and .json5 are both reported as the same language type (86 = L_JSON5), so the extension check is what actually separates them;
• with draw-on-open enabled, the .jsonc document is drawn automatically while the byte-identical content named .json5 is not;
• the rest of the jsonc assertions (snapshot caching, opt-in behaviour, manual refresh) pass unchanged.