Make org.eclipse.cdt.lsp.clangd API public

[ghentschke]

Since some vendors using the API in the org.eclipse.cdt.lsp.clangd package we should tag it as public API.

[ruspl-afed]

• -1 for LspPlugin - it shouldn't expose its UI Workbench nature to API
• -1 for LspUtils - they should be internal or we should extract required functionality to dedicated types

[ghentschke]

I am a little confused. LspPlugin and LspUtils are part of the org.eclipse.cdt.lsp package in the org.eclipse.cdt.lsp bundle.
I agree, LspPlugin and LspUtils need some rework, but this issue is about the org.eclipse.cdt.lsp.clangd package in the org.eclipse.cdt.lsp.clangd bundle. I cannot see, where the files in that package uses LspPlugin or LspUtils.

[ghentschke]

Sorry, haven't seen that LspUtils and LspPlugin gets used in the org.eclipse.cdt.lsp.clangd package.

[ghentschke]

Can be closed since #285 has been merged