fix(types): use portable string literal aliases

[oriori1703]

Summary

• use direct string literals for conform.LspFormatOpts
• make the alias compatible with both LuaLS and EmmyLua
• prevent EmmyLua from rejecting valid default_format_opts.lsp_format values

LuaLS understands both the previous nested-quote spelling and the direct literal spelling. EmmyLua only interprets the direct spelling as the actual Lua string value.

Verification

• emmylua_check 0.25.1: valid values accepted and invalid values rejected
• lua-language-server 3.19.1: valid values accepted and invalid values rejected
• Real Conform setup { default_format_opts = { lsp_format = 'fallback' } } call accepted by both analyzers
• git diff --check

make test was also run. Two unrelated newline-preservation tests fail under Neovim 0.12.5 (fuzzer formats correctly and runner e2e sets the correct output); all other tests pass. make lint could not complete because luacheck is not installed in the environment.

Related: EmmyLuaLs/emmylua-analyzer-rust#1271