Add Gleam syntax highlighting support

[anntnzrb]

Gleam currently has LSP and formatter support, but syntax highlighting is missing because the upstream tree-sitter parser doesn't publish WASM binaries.

Blocked on: gleam-lang/tree-sitter-gleam#137

Once that PR is merged and a release is published, this can be unblocked by adding a Gleam entry to parsers-config.ts with the WASM URL and nvim-treesitter queries.

[anntnzrb]

Draft PR ready: #5962