Trlc

* [LRM] The LRM for TRLC is [now written in
TRLC](https://github.com/bmw-software-engineering/trlc/tree/main/language-reference-manual). There
is also a [HTML
version](https://bmw-software-engineering.github.io/trlc/lrm.html)
available for easier reading. During conversion we fixed a number of
issues in the LRM (missing keywords, etc.) and made a few cleanups
and simplifications; however the language and its semantics have not
changed.

* [LRM, TRLC] Add support for existential quantification. This
introduces a new keywords `exists`.

* [LRM, TRLC] Also permit `in` and `not in` for arrays. This is a
convenient short-cut for existential quantification over an array to
test for membership.

* [LRM, TRLC] Fix oversight in the LRM, not allowing `true` and
`false` to appear as values.

* [TRLC] Fix ICE when attempting to parse completely empty files.

* [LRM, TRLC] Two minor improvements for the handling of triple quoted
strings: empty lines are now ignored when removing common leading
whitespace, and trailing whitespace is stripped.

* [TRLC] Implement missing feature: integer values in record
declarations can be prefixed by a unary `+` or `-`.

* [TRLC] Make parse order of files more predictable (which also makes
error messages more predictable) and not so dependent on filesystem
ordering.

* [LRM, TRLC] Support declarations of checks in the `.rsl` files. This
means you don't need to have a separate check file, allowing you to
implement your process or business logic closer to the type
declarations. Dedicated check files continue to be supported.

* [LRM, TRLC] Rework builtin functions to not have the ugly `trlc:`
prefix. Builtin functions now look more natural, e.g. `len` instead
of `trlc:len`. The legacy functions will remain as a deprecated
feature, and we'll remove them at a future date.

* [TRLC] Fix bug in array parsing and evaluation.

* [TRLC] Add the first two checks to the `--lint` option: one about
misleading unary operators, and another on using deprecated
functions.

* [TRLC] Fix a bug where a record reference to a type extension
instead of the base type was not permitted, when it should be.

* [TRLC] Add `--brief` mode intended for CI. Expand tutorial to
explain how it works.

* [TRLC] Add two new API functions (a [simpler
lookup](https://bmw-software-engineering.github.io/trlc/manual/ast.html#trlc.ast.Symbol_Table.lookup_assuming)
and [inheritence
checking](https://bmw-software-engineering.github.io/trlc/manual/ast.html#trlc.ast.Record_Type.is_subclass_of)
for record types).

* [LRM, TRLC] First public release