Introduce support for TOML config-format

Introduce support for TOML config-format

This commit introduces support for the semi-popular TOML
config-format[1] through a new major-mode: toml-ts-mode.

I've read through the full spec[2], and from what I can see this
major-mode should provide correct syntax-highligting for every sort of
config-declaration which adheres to the specification.

Besides that it also adds support for imenu and basic tree-sitter
based navigation.

[1] https://toml.io/en/
[2] https://toml.io/en/v1.0.0