'Unknown subproject' error when referencing subprojects in a non-default directory

[kosude]

Attempting to use the subproject function to reference a subproject within a directory other than subprojects/ seems to result in an error being displayed in the editor.

In other words, the extension doesn't seem to obey the value specified to the subproject_dir argument when initialising the project.

Note that when I encounter this, I don't get any actual errors at configuration, but those displayed inline by the extension remain.

[JCWasmx86]

This needs an update to the vscode extension, as 1.25.1 is using 4.2.2. This feature requires 4.3.x.

[JCWasmx86]

I created #252 with a fix that upgrades mesonlsp to 4.3.1