"Expand Selection" doesn't expand to strings or scopes

Nobody has claimed this yet.

I use VScode's "Expand Selection" command a lot to copy strings around, but when I try to use it on Lua strings, it expands the selection a lot more than just the string I'm on.

## Actual behavior

Suppose my cursor is inside the word example:

> hs.alert(prefix .. " an exam`|`ple string " .. suffix)

1st call to Expand Selection selects the word:

> hs.alert(prefix .. " an `example` string " .. suffix)

2nd call to Expand Selection selects everything between `()` instead of the full string:

> hs.alert(`prefix .. " an example string " .. suffix`)

## Expected behavior

2nd call to Expand Selection should select just the string:

> hs.alert(prefix .. `" an example string "` .. suffix)

In JavaScript, I noticed it even has two steps for the string, the inner part of the string, then the outer:

> alert(prefix + " an `example` string " + suffix)

> alert(prefix + "` an example string `" + suffix)

> alert(prefix + `" an example string "` + suffix)

That would also work!

## Similar scenarios

Scopes are also currently skipped:

> local myFunction = function () ret`|`urn true end

> local myFunction = function () `return` true end <- 1st call selects the word

> local myFunction = `function () return true end` <- expected: 2nd call selects the function scope

> `local myFunction = function () return true end` <- actual: 2nd call selects the line instead

Same for `Object.property`:

> Object.`property` = true

> `Object.property` = true <- expected

> `Object.property = true` <- actual

Start by locating the Expand Selection implementation and any tests covering selection expansion; the issue does not name specific files or entry points. Reproduce the Lua string, function-scope, and Object.property examples, then add coverage showing that successive expansions select the expected constructs.

Written by the indexing model from the issue text.