Add support for ccls?

[rqdmap]

When using symbols-outline along with ccls, errors will occur:

Error executing vim.schedule lua callback: ...lazy/symbols-outline.nvim/lua/symbols-outline/parser.lua:180: attempt to index a nil value
stack traceback:
        ...lazy/symbols-outline.nvim/lua/symbols-outline/parser.lua:180: in function 'get_lines'
        ...lazy/symbols-outline.nvim/lua/symbols-outline/writer.lua:91: in function 'parse_and_write'
        ...e/nvim/lazy/symbols-outline.nvim/lua/symbols-outline.lua:301: in function 'on_symbols'
        ...-outline.nvim/lua/symbols-outline/providers/nvim-lsp.lua:78: in function 'callback'
        /usr/share/nvim/runtime/lua/vim/lsp.lua:2021: in function 'handler'
        /usr/share/nvim/runtime/lua/vim/lsp.lua:1394: in function ''
        vim/_editor.lua: in function <vim/_editor.lua:0>

I found it's because symbols-outline doesn't support ccls's symbol kind, i.e 252, 253, 254 and 255, which can be checked in ccls source code

Will symbols-outline add support for ccls?

[elliotwesoff]

I would love to see #218 merged! I'm getting that exact error every time I work on a C/C++ file.