Template formatter with plural blocks

Implement format_template(text: str, keys: list[str], values: list[str]) -> str. keys[i] maps to values[i]; all values are strings.

text can contain two kinds of expressions in braces:

1. Placeholder {name}: replace it with the value of name. Whitespace around the name is ignored.
2. Plural block {name, plural, one{...} other{...}}: the value of name must be an integer. If it is 1, use the one branch; otherwise use the other branch. If there is no one branch, use other. Inside the chosen branch, every # becomes the number. A branch may also contain placeholders, which are expanded too.

Rules:

• Text outside expressions is copied unchanged. # outside a plural branch is a plain character.
• An expression whose name is not in keys is left in the output unchanged.
• Raise an error for an unmatched { or }, a block type other than plural, a plural block with no other branch, or a plural value that is not an integer.
• 0 <= len(text) <= 10^4.

Example 1

text   = "{n, plural, one{# layer} other{# layers}} hidden"
keys   = ["n"]
values = ["4"]
→ "4 layers hidden"

With values = ["1"] the result is "1 layer hidden".

Example 2

text   = "{owner} shared {k, plural, one{# board with {team}} other{# boards with {team}}}"
keys   = ["owner", "k", "team"]
values = ["Rosa", "1", "Design"]
→ "Rosa shared 1 board with Design"

Reconstructed from the reported question; details may differ from what interviewers ask.