Add syntax support for toolchain directive

So that this directive can be highlighted in `go.mod`

There's not much detail for the syntax of this directive in the grammar
file[1] so this implementation is based on the document linked there,
specifically[2] and glancing at some code[3]. For example support
includes versions like

* go1.21.3
* go1.20
* go1.21rc4
* go1.12.3-somesuffix
* go1.21rc4-somesuffix more stuff

But excludes e.g.:

* go1.20.3rc4
* go1.20abc

The highlight groups this syntax is linked to just follows what is done
for `gomodGoVersion`

[1] https://go.dev/ref/mod#go-mod-file-toolchain
[2] https://go.dev/doc/toolchain#version
[3] https://go.googlesource.com/go/+/165383381199de6632665f43561e8e0dfc96f067/src/cmd/go/internal/gover/toolchain.go#24

syntax/gomod.vim

@@ -11,27 +11,30 @@ syntax case match
 " https://golang.org/ref/mod#go-mod-file-grammar
 
 " match keywords
-syntax keyword gomodModule  module
-syntax keyword gomodGo      go      contained
-syntax keyword gomodRequire require
-syntax keyword gomodExclude exclude
-syntax keyword gomodReplace replace
-syntax keyword gomodRetract retract
+syntax keyword gomodModule     module
+syntax keyword gomodGo         go        contained
+syntax keyword gomodToolchain  toolchain contained
+syntax keyword gomodRequire    require
+syntax keyword gomodExclude    exclude
+syntax keyword gomodReplace    replace
+syntax keyword gomodRetract    retract
 
 " require, exclude, replace, and go can be also grouped into block
 syntax region gomodRequire start='require (' end=')' transparent contains=gomodRequire,gomodVersion
 syntax region gomodExclude start='exclude (' end=')' transparent contains=gomodExclude,gomodVersion
 syntax region gomodReplace start='replace (' end=')' transparent contains=gomodReplace,gomodVersion
 syntax region gomodRetract start='retract (' end=')' transparent contains=gomodVersionRange,gomodVersion
 syntax match  gomodGo            '^go .*$'           transparent contains=gomodGo,gomodGoVersion
+syntax match  gomodToolchain     '^toolchain .*$'    transparent contains=gomodToolchain,gomodToolchainVersion
 
 " set highlights
-highlight default link gomodModule  Keyword
-highlight default link gomodGo      Keyword
-highlight default link gomodRequire Keyword
-highlight default link gomodExclude Keyword
-highlight default link gomodReplace Keyword
-highlight default link gomodRetract Keyword
+highlight default link gomodModule     Keyword
+highlight default link gomodGo         Keyword
+highlight default link gomodToolchain  Keyword
+highlight default link gomodRequire    Keyword
+highlight default link gomodExclude    Keyword
+highlight default link gomodReplace    Keyword
+highlight default link gomodRetract    Keyword
 
 " comments are always in form of // ...
 syntax region gomodComment  start="//" end="$" contains=@Spell
@@ -45,6 +48,16 @@ highlight default link gomodString  String
 syntax match gomodReplaceOperator "\v\=\>"
 highlight default link gomodReplaceOperator Operator
 
+" match toolchain versions, per https://go.dev/doc/toolchain#version, e.g.
+"
+"  * go1.X.Y
+"  * go1.X
+"  * go1.XrcN
+"  * go1.X.Y-somesuffix
+"  * go1.XrcN-somesuffix more stuff
+syntax match gomodToolchainVersion "go1\.\d\+\(\(.\d\+\)\|\(rc\d\+\)\)\?\([ \t-].*\)\?" contained
+highlight default link gomodToolchainVersion Identifier
+
 " match go versions
 syntax match gomodGoVersion "1\.\d\+" contained
 highlight default link gomodGoVersion Identifier