Require at least one keyword for complex messages

[eemeli]

Fixes #787
Fixes #837

We currently allow complex messages to start with {{, so hello and {{hello}} mean the exact same thing. As identified in #787, when we add bidi isolation to the syntax (#811), this will mean that each of the following will be valid sequences starting a complex message:

• .
• {{
• LRI{{ or {LRI{
• RLI{{ or {RLI{
• FSI{{ or {FSI{

where LRI, RLI, and FSI represent the U+2066..U+2068 characters.

This seems like a lot, esp. when the only reason why a complex message must be valid even if it has no keywords is to allow for messages like {{.zip}}, i.e. ones starting with a literal ..

If we were to instead allow for \. as an escape, we could require complex messages to start with a keyword, and then the set of valid sequences starting a complex message reduces to only:

• .

[aphillips]

@eemeli We have design documents discussing these problems and technical discussions scheduled (including today's [2024-07-29] call). Please don't file additional PRs while we're discussing the design.

[lucacasonato]

For the record, I am relatively strongly against this. It makes refactoring from simple -> complex message more annoying, as you have to transition through an invalid / confusing state:

---

Hello, World!

Renders as Hello, World!

---

{{Hello, World!}}

Renders as {{Hello, World!}}

---

.local $foo = {1}
{{Hello, World!}}

Renders as Hello, World!

[eemeli]

Giving up on this one; not worth the fight.